"Finally, here comes omega (ω). Everybody knows it signals the end. Although we are still near the middle of the alphabet, albeit on the downhill side, people are noticing that Greek has two 'O's."

Writes Mary Norris, in "A Linguistic Look at Omicron/What is this penchant for using Greek to designate disasters?" 

Omega means “big O.” (Get it? O-mega?) Omicron means “little O.” O-micron. The Greek micro, or “small,” has given us “microbes,” “microscopes,” and “microminis.” Mega, Greek for “big,” has come into English in “Mega Millions,” “mega-threat,” and “megalopolis.” If the repercussions from little old omicron have been so catastrophic, what are we to expect from big bad omega? 

Before panic sets in, a note on pronunciation: “small O” and “big O” refer less to the size and shape of the written letters (omega is an omicron with big feet: Ω) than to their sounds. They are vowels with short and long values. Though there is no universal agreement about it, many American classicists pronounce omicron with a short “o,” as in “om,” and omega with a long “o,” like an Irish surname: O’Mega. 

Norris links to a Mother Jones article from a few weeks ago, "I Asked Seven Classics Experts How to Say 'Omicron.' Come Down the Rabbit Hole With Me." Excerpt:

My first call was to David Sider, an American scholar with a terrific Bronx accent, in whom I found an equally frustrated ally. “You’ve called the right person!” he exclaimed. “I was driving in a car the other day, listening to the radio, and hit two different people on two different episodes say ‘ah-ma-CRON. And that’s wrong.”

“Kind of like the French president?” I asked. “Ah, Macron!”

“Yes, yes, exactly,” he said. “I’ve been bothered.”