I'm not entirely sure what sector Camden Yards is in, but as long as you have a digital scanner, you should be able to monitor the City trunk for that area.
Note however that there is a report that the city is in the midst of rebanding - right now I can monitor with the 396 with few problems, but if they do switch frequencies - and I think they're going to have to - that will cause some headaches, at least until the firmware fixes are in place.
I'm pretty sure we have Orioles freqs in our wiki, and I'll have to see about Camden Yards. Would be a good time to use Signal Stalker or similar... Eric Carlson's website is rather old, but it does suggest some freqs to check, to see if they're even being used, or have new users on them. Remember to use a short stubby type antenna so you aren't overloaded with other sources outside the Yard.
73 Mike
[edit] Here we go...(anything in blue is a link) for the Orioles...
http://wiki.radioreference.com/index.php/Major_League_Baseball_(MLB)
And a couple for Camden Yards...
http://www.radioreference.com/apps/db/?aid=2901
http://www.davidschoenberger.net/scanning/maryland/baltimorecity.htm
Of course, vendors come and go, so any vendor list is likely to change from one year to the next, depending, I suppose, with contracts with the Orioles and the owners of the Yard.
If you find new freqs, please be sure to post them. We can easily absorb them either as a wiki article or a database entry