If you are using Lion or Mountain Lion then just press and hold the 'o' key and a list of o's with accents will appear with a number underneath. Then just type the number for the character you want or click on it. Just like the iPhone works.

![Screenshot][1]

You can also use the procedure from [this other question][pah] to add a theta character to the press-and-hold menu.

  [1]: https://i.sstatic.net/Hrkdi.png
  [pah]: http://apple.stackexchange.com/questions/20505/how-to-add-characters-to-the-press-and-hold-character-picker-in-os-x-lion