Just my two cents worth. Start off by checking ALL of the fuses. Don’t just look at them from above. Pull and inspect each fuse one at a time.
It is possible that you pinched a wire re-installing your transmission. The O2 wiring harness is long and easy to get caught between the motor/transmission.
Next, check for a PATS problem (passive anti theft system). If the system is active, you should see the PATS light flashing on the dash.
Did you have any other problems with the transmission install that might shed some light on the problem?