ADC driver design for oversampling
You design the buffer amplifier to fit your signal, not the sampling rate of the ADC. But the buffer's output impedance has to match the ADC's input impedance requirements at the sampling frequency.

As a side note: Oversampling will reduce noise (which the STM32 ADCs have a lot of) but not the non-linearities of the ADC. You will still get just 12bit resolution, but with less noise.