As far as their pronunciation is concerned, none. "Ο", however, can be part of the diphthongs "-οι-", which makes an "i" sound, and "-ου-", which makes a "u" sound.
The most fundamental orthography rule in the usage of "ω" is that, whenever a verb ends in an "-o" sound (that is, in the 1st person singular), it's always spelled with an "ω" ;)
First of all see here for lots of helpful links.
These links will not only show you how to get the Greek keyboard but also how to find the Greek letters on it, how to add accents etc. https://www.duolingo.com/comment/22040507
Greek on Android and iOS! https://www.duolingo.com/comment/20607862
This σ is used at the start or in the word and this one is only used at the end ς. They sound exactly the same.
See here for other hints to make learning easier.
TIPS TO HELP YOU LEARN + HOW TO REPORT A PROBLEM