does OM have an ordinary meaning
Although it's hard to prove a negative, people don't explain its "ordinary meaning" when they explain it, so I assume the answer is "no".
I guess it has two actual meanings.
The first "actual meaning" IMO is that people explain that it has three syllables (or four if you count the silence afterwards); and, that each of the three syllables refers to different things ... but, there are diverse explanations of what those three things are.
So the symbolic meaning seems to be "three syllables representing three things" -- but (importantly) what those three things are, what the symbol represents, differs.
So (perhaps like other words, karma for example) it's a word repurposed by Buddhism. If it has a different meaning in Buddhism, I don't know whether you'd want to think of it as the same word.
The second "actual meaning" of it is (or comes from) where and how it's used, its positional or contextual meaning -- i.e. it's used to begin mantras. I think that, in the real world, people tend to learn words from hearing them in place, in context, in practice (and not from dictionary definitions). So the basic meaning of "om" is "here starts a mantra" and/or "prepare to be conscious of a mantra".
It's hard to think of an equivalent in English ... possibly "Amen" which has an overtly religious origin but may possibly be used in other contexts to mean "I solemnly agree with what was just said."
Perhaps this could be called a "formulaic" meaning -- possibly for example like:
- "Evaṃ me sutaṃ" -- prepare yourself to hear a sutta
- "Once upon a time" -- prepare yourself to hear a children's story
- "Dear sir" -- prepare yourself to read a business letter
- Ringing a little bell at the beginning of a period of silence