I have no idea idea what you mean when you say this issue is not an HOA issue but rather "a trespass issue." He never trespassed anywhere, and he was (purportedly) banned from the amenities by the HOA Board. How on earth could it be "a trespass issue" and "not an HOA issue"?? You've lost me there.

As for your theory of the OPA being like any other private property owner who can remove people from their property -- The OPA isn't like any other private property owners. The OPA is an incorporated association of members, all of whom have legally enforceable/contractual rights, set forth in recorded instruments, including in regard to use of amenities. The Board has certain powers and lacks powers not expressly granted to it in the governing documents. 

Hypotheticals can be useful to make a point. Imagine an argument occurring between a condo unit owner and its Board president, on the grounds of the condominium property in Ocean City. Now imagine a Board majority (including the Board president) banning the unit owner, summarily and without any hearing or other process, from the hallway outside his unit, because the Board president's unit is on the same hallway (and he feels unsafe); or banning the unit owner from the condo parking lot. Or Condo docks where the unit owner keeps a boat in his slip. Or all of those places. Under your way of thinking, this would be legally sound, right? No.  

And the notion that the OPA banning Tom equates to the OPA "exercising Maryland trespass laws"? I confess I don't know what you mean. The Board majority never "exercised any trespass laws," whatever that means to you. The Board majority purported to exercise power it does not have to ban a member from amenities, based on conduct of the member that it found objectionable, involving interactions with one of the Board majority. Had the ban not been set aside by injunction, and Tom had violated it in the eyes of the Board majority, I suppose the Board majority (and perhaps OPA police at the Board's direction) would have treated Tom as a trespasser. But none of that occurred, thankfully.