[...]Note that the Nationals outdrew the Orioles in 2010-2011.[...]
...and although the Nats are trailing right now (on the basis of just six home fixtures played by each team) they'll probably outdraw them this season too, once the Caps are done and we get better-drawing opponents than the Reds, Astros and Marlins in here (and of course assuming the Nats continue their current fine form, or something close to it). Even the visits to Preparation H Park At Camden Yards by the Yankees and Red Sox aren't the guaranteed 30k+ gates they once were.