Basic Spellings:
* o (as in "go", "no", "so")
* oa (as in "boat", "coat", "road")
* oe (as in "toe", "hoe", "doe")
* ow (as in "cow", "now", "how")
* o-e (as in "hope", "home", "bone")
Less Common Spellings:
* eau (as in "beau", "beautify", "plateau")
* ew (as in "sew", "few", "new")
* ough (as in "though", "dough", "through")
* au (as in "caught", "taught", "daughter")
* aw (as in "draw", "claw", "straw")
Other Notes:
* Double "o" can sometimes make a long o sound, but it's more common for a short o sound (as in "look", "book").
* "y" can sometimes make a long o sound at the end of a word (as in "baby", "happy", "pretty").
Remember that the spelling of a word can be influenced by its etymology (origin). For instance, "sew" and "few" use "ew" for a long o sound because of their Old English roots.
This is not an exhaustive list, but it covers the most common spellings of the long o sound in English.