I replaced an old O2 sensor (bad) with a new one and now the new O2 sensor takes a while to heat up and it is oscillating on the lean side. Could this be a wiring issue? Bad ground?

 

How fast is it oscillating? 8 - 10 a second is correct. As for it being on the lean side you need to go elsewhere. Ask yourself, What would make it run lean other than the computer. Vac leak, low fuel pressure, Stuck injector, bad air temp sensor, bad coolant temp sensor, etc...

I shouldn't say bad but out of calibration is a better word for it.