diff --git a/src/config/initializer.test.ts b/src/config/initializer.test.ts
index e1184f2..0118d3d 100644
--- a/src/config/initializer.test.ts
+++ b/src/config/initializer.test.ts
@@ -236,4 +236,58 @@ describe('initializer', () => {
       expect(validateConfig(loadedConfig)).toBe(true)
     })
   })
+
+  describe('oh-my-opencode optional preset', () => {
+    test('validateConfig returns true for preset without oh-my-opencode key', () => {
+      const config: ModeSwitcherConfig = {
+        currentMode: 'performance',
+        showToastOnStartup: true,
+        presets: {
+          performance: {
+            description: 'High-performance models for complex tasks',
+            model: 'anthropic/claude-sonnet-4',
+            opencode: {
+              build: { model: 'anthropic/claude-sonnet-4' },
+            },
+            // Note: no 'oh-my-opencode' key
+          },
+        },
+      }
+      expect(validateConfig(config)).toBe(true)
+    })
+
+    test('validateConfig returns true for config where some presets have oh-my-opencode and some do not', () => {
+      const config: ModeSwitcherConfig = {
+        currentMode: 'performance',
+        showToastOnStartup: true,
+        presets: {
+          performance: {
+            description: 'With oh-my-opencode',
+            opencode: {},
+            'oh-my-opencode': { agents: {} },
+          },
+          economy: {
+            description: 'Without oh-my-opencode',
+            opencode: {},
+            // Note: no 'oh-my-opencode' key
+          },
+        },
+      }
+      expect(validateConfig(config)).toBe(true)
+    })
+
+    test('preset without oh-my-opencode key has undefined value for that field', () => {
+      const preset: ModePreset = {
+        description: 'No oh-my-opencode',
+        model: 'anthropic/claude-sonnet-4',
+        opencode: {
+          build: { model: 'anthropic/claude-sonnet-4' },
+        },
+        // Note: no 'oh-my-opencode' key
+      }
+
+      // Accessing undefined key should return undefined (not throw)
+      expect(preset['oh-my-opencode']).toBeUndefined()
+    })
+  })
 })
diff --git a/src/config/initializer.ts b/src/config/initializer.ts
index 441b8fd..b7c9a54 100644
--- a/src/config/initializer.ts
+++ b/src/config/initializer.ts
@@ -60,6 +60,10 @@ function applyEconomyModel(
  * files and preserves their entire structure as-is. The hierarchical
  * structure (agent, agents, categories, etc.) is maintained exactly.
  *
+ * If `oh-my-opencode.json` does not exist (i.e., the user does not use the
+ * oh-my-opencode plugin), the `'oh-my-opencode'` key is omitted from the
+ * returned preset entirely.
+ *
  * @returns Promise resolving to a ModePreset with performance-oriented models
  * @example
  * ```typescript
@@ -86,7 +90,7 @@ async function buildPerformancePreset(): Promise<ModePreset> {
     description: 'High-performance models for complex tasks',
     ...(globalModel && { model: globalModel }),
     opencode: opencodePreset,
-    'oh-my-opencode': ohMyOpencodePreset,
+    ...(ohMyOpencodeConfig && { 'oh-my-opencode': ohMyOpencodePreset }),
   }
 }
 
@@ -98,6 +102,10 @@ async function buildPerformancePreset(): Promise<ModePreset> {
  * `opencode/glm-4.7-free` model. The hierarchical structure is
  * preserved while model values are updated recursively.
  *
+ * If `oh-my-opencode.json` does not exist (i.e., the user does not use the
+ * oh-my-opencode plugin), the `'oh-my-opencode'` key is omitted from the
+ * returned preset entirely.
+ *
  * @returns Promise resolving to a ModePreset with economy-oriented models
  * @example
  * ```typescript
@@ -125,7 +133,7 @@ async function buildEconomyPreset(): Promise<ModePreset> {
     description: 'Cost-efficient free model for routine tasks',
     model: DEFAULT_ECONOMY_MODEL,
     opencode: opencodePreset,
-    'oh-my-opencode': ohMyOpencodePreset,
+    ...(ohMyOpencodeConfig && { 'oh-my-opencode': ohMyOpencodePreset }),
   }
 }
 
diff --git a/src/config/types.ts b/src/config/types.ts
index 7373fad..60d729f 100644
--- a/src/config/types.ts
+++ b/src/config/types.ts
@@ -33,12 +33,15 @@ export type HierarchicalPreset = Record<
  * Both opencode and oh-my-opencode use the same HierarchicalPreset type,
  * allowing them to have arbitrary nested structures that are handled
  * uniformly by recursive merge functions.
+ *
+ * The `oh-my-opencode` field is optional to support users who do not use the
+ * oh-my-opencode plugin. When absent, all oh-my-opencode related processing is skipped.
  */
 export interface ModePreset {
   description: string
   model?: string
   opencode: HierarchicalPreset
-  'oh-my-opencode': HierarchicalPreset
+  'oh-my-opencode'?: HierarchicalPreset
 }
 
 /**
diff --git a/src/modes/manager.test.ts b/src/modes/manager.test.ts
index f7d6c64..338168d 100644
--- a/src/modes/manager.test.ts
+++ b/src/modes/manager.test.ts
@@ -95,7 +95,7 @@ class MockModeManager {
       )
     }
 
-    if (this.ohMyConfig) {
+    if (this.ohMyConfig && preset['oh-my-opencode']) {
       deepMergeModel(
         this.ohMyConfig as Record<string, unknown>,
         preset['oh-my-opencode']
@@ -130,6 +130,7 @@ class MockModeManager {
     // Check oh-my-opencode: recursively
     if (
       this.ohMyConfig &&
+      preset['oh-my-opencode'] &&
       hasDriftRecursive(
         this.ohMyConfig as Record<string, unknown>,
         preset['oh-my-opencode']
@@ -188,7 +189,9 @@ class MockModeManager {
       : 'Global model: (not set)'
 
     const opencodeTree = formatHierarchicalTree(preset.opencode)
-    const ohMyOpencodeTree = formatHierarchicalTree(preset['oh-my-opencode'])
+    const ohMyOpencodeTree = preset['oh-my-opencode']
+      ? formatHierarchicalTree(preset['oh-my-opencode'])
+      : ''
 
     return [
       `Current mode: ${currentMode}`,
@@ -222,7 +225,9 @@ class MockModeManager {
     }
 
     // Simulate updating oh-my-opencode.json
-    if (this.ohMyConfig) {
+    if (!preset['oh-my-opencode']) {
+      results.push('oh-my-opencode.json: skipped (not configured)')
+    } else if (this.ohMyConfig) {
       results.push('oh-my-opencode.json: updated')
     } else {
       results.push('oh-my-opencode.json: skipped (not found)')
@@ -657,4 +662,103 @@ describe('ModeManager', () => {
       expect(manager.lastDriftToast).not.toBeNull()
     })
   })
+
+  describe('oh-my-opencode optional (no oh-my-opencode key in preset)', () => {
+    /**
+     * A config without 'oh-my-opencode' key in the preset, simulating
+     * a user who does not use oh-my-opencode.
+     */
+    function cloneConfigWithoutOhMy(): ModeSwitcherConfig {
+      return {
+        currentMode: 'performance',
+        showToastOnStartup: true,
+        presets: {
+          performance: {
+            description: 'High-performance models for complex tasks',
+            model: 'anthropic/claude-sonnet-4',
+            opencode: {
+              build: { model: 'anthropic/claude-sonnet-4' },
+              plan: { model: 'anthropic/claude-sonnet-4' },
+            },
+            // Note: no 'oh-my-opencode' key
+          },
+          economy: {
+            description: 'Cost-efficient free model for routine tasks',
+            model: 'opencode/glm-4.7-free',
+            opencode: {
+              build: { model: 'opencode/glm-4.7-free' },
+              plan: { model: 'opencode/glm-4.7-free' },
+            },
+            // Note: no 'oh-my-opencode' key
+          },
+        },
+      }
+    }
+
+    test('switchMode returns "skipped (not configured)" for oh-my-opencode when preset has no oh-my-opencode key', async () => {
+      manager.setConfig(cloneConfigWithoutOhMy())
+      manager.setOpencodeConfig(sampleConfigs.opencodeConfig)
+      manager.setOhMyConfig(sampleConfigs.ohMyOpencodeConfig)
+      await manager.initialize()
+
+      const result = await manager.switchMode('economy')
+      expect(result).toContain('oh-my-opencode.json: skipped (not configured)')
+    })
+
+    test('getStatus does not crash when preset has no oh-my-opencode key', async () => {
+      manager.setConfig(cloneConfigWithoutOhMy())
+      await manager.initialize()
+
+      const status = await manager.getStatus()
+      expect(status).toContain('Current mode: performance')
+      expect(status).toContain('Oh-my-opencode config:')
+      expect(status).toContain('(none configured)')
+    })
+
+    test('initialize does not crash on drift check when preset has no oh-my-opencode key', async () => {
+      const config = cloneConfigWithoutOhMy()
+      config.currentMode = 'economy'
+      manager.setConfig(config)
+      // opencode drifted, oh-my-opencode present in FS but preset has no oh-my-opencode key
+      manager.setOpencodeConfig({
+        model: 'anthropic/claude-sonnet-4', // mismatch with economy
+        agent: {
+          build: { model: 'anthropic/claude-sonnet-4' },
+          plan: { model: 'anthropic/claude-sonnet-4' },
+        },
+      })
+      manager.setOhMyConfig(sampleConfigs.ohMyOpencodeConfig)
+
+      // Should not throw
+      await manager.initialize()
+
+      // Drift detected via opencode config, so toast should be set
+      expect(manager.lastDriftToast).not.toBeNull()
+      expect(manager.lastDriftToast).toContain('economy')
+    })
+
+    test('initialize does not apply oh-my-opencode changes when preset has no oh-my-opencode key', async () => {
+      const config = cloneConfigWithoutOhMy()
+      config.currentMode = 'economy'
+      manager.setConfig(config)
+      manager.setOpencodeConfig({
+        model: 'opencode/glm-4.7-free', // already matching
+        agent: {
+          build: { model: 'opencode/glm-4.7-free' },
+          plan: { model: 'opencode/glm-4.7-free' },
+        },
+      })
+      manager.setOhMyConfig({
+        agents: {
+          sisyphus: { model: 'anthropic/claude-sonnet-4' }, // different model
+        },
+      })
+
+      await manager.initialize()
+
+      // No drift (opencode matches), so toast should not be set
+      // and oh-my-opencode should NOT be modified (preset has no oh-my-opencode)
+      expect(manager.lastDriftToast).toBeNull()
+    })
+  })
 })
diff --git a/src/modes/manager.ts b/src/modes/manager.ts
index dbba430..bfe1490 100644
--- a/src/modes/manager.ts
+++ b/src/modes/manager.ts
@@ -259,7 +259,9 @@ export class ModeManager {
 
     // Apply the preset to actual config files
     await this.updateOpencodeConfig(preset.model, preset.opencode)
-    await this.updateOhMyOpencodeConfig(preset['oh-my-opencode'])
+    if (preset['oh-my-opencode']) {
+      await this.updateOhMyOpencodeConfig(preset['oh-my-opencode'])
+    }
 
     // Notify user to restart (fire-and-forget to avoid blocking
     // plugin initialization when UI is not yet ready).
@@ -288,6 +290,10 @@ export class ModeManager {
    * Checks global model and per-agent model values recursively. Returns true
    * if any expected value differs from the actual file content.
    *
+   * If the preset does not include an `'oh-my-opencode'` key (i.e., the user
+   * does not use the oh-my-opencode plugin), drift checking for that config
+   * file is skipped entirely.
+   *
    * @param preset - The mode preset to compare against
    * @returns True if actual configs differ from the preset
    * @private
@@ -317,9 +323,10 @@ export class ModeManager {
       return true
     }
 
-    // Check oh-my-opencode: recursively check
+    // Check oh-my-opencode: recursively check (skip if preset has no oh-my-opencode config)
     if (
       ohMyConfig &&
+      preset['oh-my-opencode'] &&
       hasDriftRecursive(
         ohMyConfig as Record<string, unknown>,
         preset['oh-my-opencode']
@@ -437,8 +444,10 @@ export class ModeManager {
     // opencode: recursively format tree
     const opencodeTree = formatHierarchicalTree(preset.opencode)
 
-    // oh-my-opencode: recursively format tree
-    const ohMyOpencodeTree = formatHierarchicalTree(preset['oh-my-opencode'])
+    // oh-my-opencode: recursively format tree (skip if not configured)
+    const ohMyOpencodeTree = preset['oh-my-opencode']
+      ? formatHierarchicalTree(preset['oh-my-opencode'])
+      : ''
 
     return [
       `Current mode: ${currentMode}`,
@@ -459,7 +468,7 @@ export class ModeManager {
    * This method performs the following operations:
    * 1. Validates that the requested mode exists
    * 2. Updates `opencode.json` with new global model and agent settings
-   * 3. Updates `oh-my-opencode.json` with new agent settings
+   * 3. Updates `oh-my-opencode.json` with new agent settings (skipped if preset has no `'oh-my-opencode'` key)
    * 4. Updates `agent-mode-switcher.json` with the new current mode
    * 5. Shows a toast notification (if available)
    *
@@ -470,6 +479,7 @@ export class ModeManager {
    * @returns Promise resolving to a formatted result message with status of each config update
    * @example
    * ```typescript
+   * // With oh-my-opencode configured:
    * const result = await manager.switchMode('economy');
    * console.log(result);
    * // Output:
@@ -482,6 +492,18 @@ export class ModeManager {
    * //   - agent-mode-switcher.json: updated
    * //
    * // Note: Restart opencode to apply changes.
+   *
+   * // Without oh-my-opencode configured:
+   * // Output:
+   * // Switched to economy mode
+   * // Cost-efficient free model for routine tasks
+   * //
+   * // Results:
+   * //   - opencode.json: updated
+   * //   - oh-my-opencode.json: skipped (not configured)
+   * //   - agent-mode-switcher.json: updated
+   * //
+   * // Note: Restart opencode to apply changes.
    * ```
    */
   async switchMode(modeName: string): Promise<string> {
@@ -502,10 +524,10 @@ export class ModeManager {
     )
     results.push(`opencode.json: ${opencodeResult}`)
 
-    // 2. Update oh-my-opencode.json directly (agents section only)
-    const ohMyResult = await this.updateOhMyOpencodeConfig(
-      preset['oh-my-opencode']
-    )
+    // 2. Update oh-my-opencode.json directly (agents section only, skip if not configured)
+    const ohMyResult = preset['oh-my-opencode']
+      ? await this.updateOhMyOpencodeConfig(preset['oh-my-opencode'])
+      : 'skipped (not configured)'
     results.push(`oh-my-opencode.json: ${ohMyResult}`)
 
     // 3. Update plugin configuration