How would you abbreviate surnames starting with Mc/O/D?

Your last initial is your last initial. The only situation where this becomes relevant is if there are 2 John Ms on the team. Then it could be very useful to abbreviate one John McD. vs plain John M.

I do wonder about Dutch "van" names, like Greta van Susteren. I could very easily see this abbreviated as GvS.

I live in Ireland. The simple last initial form is the most commonly used:

McDonald > M.
MacDonald > M.
Macdonald > M. (this is a Scottish name)
O'Brien > O.

The other option (which I prefer) is to include the entire surname up to the second capital letter, thus:

McDonald > McD.
MacDonald > MacD.
Macdonald > M. (no change there)
O'Brien > O'B.

I've never seen

McDonald > MD
MacDonald > MD

You might see

O'Brien > OB

but that's probably just people having trouble with apostrophes.

Some people introduce spaces and write O'Brien as O Brien, MacDonald as Mac Donald, and McDonald as Mc Donald. This seems iffy to me in English, though the prefixes do derive from independent Irish words. But if a man is going to give his surname as Ó Briain, I would expect his sister's surname to be Ní Briain.

Since using the first name + last name initial is the convention chose by your team, you should also agree on what the convention should be for these surnames on your question. As long as the chosen convention is followed by everybody, it should work for you guys, shouldn't it?