The most likely candidates to make an appearance on the team were fairly easily identified as those already on the team's 40 man roster. Since those posts and from the guys who were on the 40 man at that time, Oliver Drake, Tyler Wilson, Mike Wright, & Rey Navarro have all made their Orioles debuts. In addition to those guys, Chaz Roe, Cesar Cabral, and Chris Parmelee have also been added to the 40 man and have made their Orioles debuts.
Still remaining on the 40 man roster, and having not yet made their Orioles debuts are:
and knuckleballer Eddie Gamboa, who is 4-4 with a 3.45 ERA over 11 starts at AAA Norfolk.
Of those two, I would say that Gamboa is a strong candidate to make his Orioles debut in the relatively near future, especially considering the O's already called him up to Baltimore once earlier in the season but he didn't make it into a game at the time. Conversely, Berry is a candidate to be removed from the 40 man and I don't think we will see him in Baltimore anytime soon.
The O's have also added reliever Jorge Rondon on a waiver claim from Colorado in early May and he now resides on the 40 man roster as well. He has pitched in 12 games for the Tides and is 2-0 with a 2.89 ERA so he is certainly a viable candidate to make an appearance for the O's.
Beyond those three members of the 40 man who haven't yet played for the O's, I would say the likely or possible members of the Tides to make it to Baltimore are, in no particular order, Dariel Alvarez, Paul Janish, Julio Borbon, Pedro Beato, Michael Bowden, Zach Davies, and Pat McCoy. Prospect types like Alvarez and Davies will almost certainly play for the O's at some point, barring a trade or an injury, but who knows when that could be? Beato, Bowden and McCoy are all pitching well for the Tides, and I know that Buck seems to like the defense of Janish and Borbon, so that keeps them as possibilities.
There are obviously thousands of other players currently playing for the other 29 MLB teams and their affiliates that could come to the O's in some sort of trade, but I just can't assess the likelihood that any of them come to Baltimore and appear for the Birds.
After all of that analysis, here's my final prediction for Oriole #1000...Eddie Gamboa! Time will tell if I'm right, but I would expect #1000 to occur within the next month-ish, so the wait shouldn't be too long! Let me know who you think #1000 could be in the comments.