O'Mahony isn't eligible.
The outfield U-21s out in Greece who have been pictured are Ben Jackson, Leigh Kavanagh, Jack Hinchy, Samy Chouchane and Josh Duffus (well, plus Jack Hinshelwood of course, but I think we have to call him first-team squad at this point).
We really shouldn't. In order to be eligible to appear on the B squad list a player has to have been with the club for two years (or three if they have had a spell out on loan). O'Mahony only joined the club in January.
Casper Nilsson is the only recognised full-back from the development squad who could be named in the squad (and he is a right-back, although he has played as a left-back on occasion).
He certainly didn't when Walsall scored their second in the EFL Trophy - to the extent that I felt the need to post THREE screenshots of his extraordinary "defending".