'97 buick century custom 164,166 miles why oil in coolant resevor not losin oil

Probably not engine oil,more than likely it's Transmission Fluid. The Radiator is damaged internally and your transmission fluid is now mixing with the engine coolant.the radiator has transmission lines going into it from the transmission,so the radiator can cool the transmission fluid as well as the engine coolant. Make sure it's not transmission fluid. If you have a look at the radiator and you'll notice the radiator has two metal pressure lines on the same side one near the top and on near the bottom of the assembly. These lines come from the transmission to the transmission cooler that is integrated into the radiator. If it is transmission fluid then the vehicle will have shifting issues because the coolant would also get into the transmission causing the clutches and torque converter to overheat