I have a Cel #41(OBD1), would it be best to replace the o2 sensor and heater? Where is a how-to on replacing them(I already scoured google)

The engine has trouble starting up when humid/cold, could this be due to the distributor(it looks out of shape and seeps oil from the o-ring), or is it likely the o2 sensor problems im having?

Thanks in advance

 

Completely stock mechanically and has an Auto trans. I am the second owner at 140k miles.