Can someone show me a good article explaining the use of the Ο (omikron) and Ω?
It's like asking about using "ee" versus "ea" in English.
The spellings are different because the sounds used to be different -- and both English and Greek use an etymological spelling, writing things they used to be written even if the pronunciation has merged.
But unless you speak Old English or Attic Greek, that doesn't help you a lot, and it's just a matter of memorisation.
For example, ο χώρος "the space" and ο χορός "the dance" sound exactly identical except for the position of the accent, and there's nothing that could let you guess that one is written with omega and the other with omicron.
So it's like learning that "encounter" is spelled "meet" but "flesh" is spelled "meat" -- you just have to memorise that those two identically-pronounced words are spelled differently (and which one is spelled which way!).