You must wait for 6 months before attending another OD/AD. I went to 2 different dates a week apart, and at the second one the recruiter told me I wasn't supposed to be there and that I needed to wait.
That being said...She let me stay for the second day since I had traveled quite a ways...and I made it to the final interview. Now I'm just waiting to hear if they want me.
So just be aware that if you spend the money to travel someplace for a second OD, there is a chance they may just turn you away.
Good luck! Make sure you keep trying!
K