That bar is there (I'm sure it needs to me for safety reasons), but it didn't bother me, and I'm sure if you'd be be in the 2nd row of the mezz instead of the first it wouldn't be an issue at all (I was in the 3rd for Therese Raquin and it didn't obstruct anything from there).
I love we me front mezz seat, and would always choose that over further back in the Orchestra, but that's a bit of personal preference.
I wouldn't go back all the way to row O if you can sit closer, and I wouldn't pick the side Orch seats if you can sit in the center, esp. since those are pretty far to the side, not on the isle, right? Out of the options you mentioned I would go with row A. There seemed to be a fair amount of space between the front row and the stage, and it's not terribly high from what I could tell from the mezz (I've actually never sat downstairs in that theater), so your view should be fine. But being that close is also a matter of preference...