1) Why use an opamp when a single transistor can make a 40KHz oscillator?
2) Why must it be a Cmos opamp?
3) Have you ever seen how complicated the circuit inside an opamp is?
4) Why try to design and make an opamp when you can just buy one cheaply?
You don't want to design a Cmos opamp. Your teacher told you to design an oscillator circuit using a Cmos opamp and you mis-understood.