How can I determine the capacitance required for an oscillator (NOT A CRYSTAL!)?
No, you need no capacitor.
Just connect the external (crystal or whatever) oscillator output to the oscillator input of the CPU (OSC1 or OSC2; or maybe called XTAL1 or XTAL2) and leave the other terminal unconnected. Only one of them is input and you need to find out which one of them by looking into the datasheet. I'm sure the datasheet tells you how to connect the external clock.

Here is an example from the ATmega32 datasheet: