Add keyboard shortcut to print custom characters
There's the default long press functionality, where you long press a letter (~2sec) and it gives you a tooltip with alternatives that are... I guess similar to that letter. You can then press the associated numbers to replace the original letter. 



You can also navigate the list with:


TAB and Shift + TAB
and
← and → 
Pressing Enter will replace the original letter.
Pressing Esc will cancel




Example where I long pressed s

 



Example where I long pressed o:

I think it doesn't give me the ö, because that already exists in my keyboard layout as its own key.





You can adjust the delay for when the tooltip shows up here:

System Preferences &gt; Keyboard &gt; Keyboard (first tab) &gt; Delay Until Repeat

When Delay Until Repeat is set to Short, the delay is maybe half a second, if I had to guess.