What is opt-cmd-] bound to and how can I switch this off?
Shortcut Detective from Irradiated Software's labs page should tell you exactly what is picking up this keypress.
ShortcutDetective detects which app receives a keyboard shortcut (hotkey).
I would go into system preferences > keyboard > shortcuts, and browse through all of the keyboard shortcuts and see if any have ⌘⌥]as they’re keybind. you could also open Automator, and press the record button, press ⌘⌥] and then click stop recording and see what it says that you did.