I noticed that some condenser microphones use an ocsilator to power the capsule and others just use the phantom power. Why is the ocsilator needed?

 

There are two types of condenser microphones. One has a DC voltage bias and the other has an RF signal applied across it. See this for more details;

https://www.streetdirectory.com/tra...nd_functioning_of_a_condenser_microphone.html