In Linux you can use ComposeKey, setting it for example (in System settings) as RightCtrl (the right-Ctrl key). RightCtrl is better than AltGr in Italian keyboard, to keep AltGr for some characters like ‘[‘, or ‘]’, or ‘@’, or ‘#’, otherwise unaccessible.
In that way, when you type 1) first RightCtrl 2) then ^ 3) then o, you will get ô. You don’t need to press simultaneously all the keys.
To sum up, the main simbols :
- RightCtrl+^+o = ô
- RightCtrl+”+o = ö
- RightCtrl+’+o = ó