The only pronunciation I’ve heard for this letter of the Greek alphabet since it was used for the name of the latest Coronavirus variant stresses the first letter, which is being pronounced to rhyme with both ohm and Tom indiscriminately.
And yet the name ‘means’ “little O” as opposed to “big O” which is omega.
If we split the names of both letters into their constituent parts, then the pronunciation is clearly ‘O micron’ and ‘O mega’, but when the two words are fused into one, the traditional English stress rule comes into play, and ‘O mega’, stressed (as two words) with a main stress on both halves, changes its stress to the first syllable of the three, because a short vowel in a penultimate syllable causes the stress to move back to the antepenulitmate, in this case initial, syllable, and we get OM-e-ga (capitals indicate the stressed syllable – I’m using a re-spelling rather than phonetic transcription to allow for variations in the vowel qualities). All three current English pronunciation dictionaries (Oxford, Cambridge and Longman) give this as the principal British English pronunciation, but allow that o-MEGGa is the American version.
When we come to omicron as a single word, the other part of the rule applies: a long vowel in the penultimate syllable retains the stress. As we all know, the word micron has a ‘long’ -i- (actually, of course, a diphthong) – cf microphone, micrometer, microscope, so the pronunciation of omicron ‘ought to be’ with the same vowel: o-MY-kron. This is the first pronunciation given by the same three dictionaries, but allowing OHM-i-kron or OMM-i-kron as less common alternatives.
In my view, the English stress rule is breaking down at the moment (changing might be a better word to use if I could be sure in which direction it is moving), and presumably by analogy with omega‘s initial stress, omicron is shifting to initial stress as well.
Boris Johnson prides himself on his Classical scholarship, but even he was using initial stress in his news conference on 27 November 2021, so I think that any attempt to rein this in, and return to the traditional second syllable stress with a ‘long’ I, is doomed.