This happened to my Bravada, though it might not be specific to Oldsmobile's, and further defining my condition, which started after a windshield replacement. What happened to me was, that under the "A" pillar trim, a series of screws had not been replaced after the new glass was installed. My leakage happened after a rain or washing the car. Maybe this will help.
Secondarily, and this happens mostly in the summer (or when the A/C is creating a lot of condensate); if the drain tube gets clogged, the air-handler can become full of water and soak the passenger carpet. This usually happens to the front passenger side first.