The Internet is your friend.
From the A's website:
http://oakland.athletics.mlb.com …rk/oak_ballpark_guide.jsp
Cameras & Videos:
Cameras and videos are allowed in the Stadium during all regular season baseball games as long as they are not for commercial use. Both cameras and videos must be hand held; tripods cannot be set up in the seating areas or the aisles (monopods are allowed), and must not block the view of the ball game for other guests.
The more important question is will you be close enough to get anything worthwhile, and with the relatively small aperture of your lens, will the light be decent enough.
I have shot a numerous MLB parks with pro gear, and I'll tell you what - the lighting at night has barely been good enough for me to get 1/640s at f/2.8 and ISO1600, and sometimes I'll even have to bump it to ISO3200.
Another web site says the light is 1/500th @ F: 2.8 @ 1600 so since you don't have an f/2.8 lens and can only go to ISO 1600 you will be shooting at 1/250 or slower and this won't stop any action.
So, my recommendation is take a point and shoot and take some pictures of you and your party and leave your XTi etc at home.