Transmission or what?

i have a 1990 buick century and have been driving it in overdrive since i inherited it in April. just about a week or so ago, my transmission started messing up. i checked the fluid and it was fine, so i started driving it in just regular drive. since I've been doing that, the transmission hadn't been bothering me. but today, i drove it in overdrive again, and the transmission slipped again! why is it only doing so in overdrive and not drive?