It is possible that the oil pressure sending unit was inadvertently disconnected. Verify that the connector is in place.
It is also possible, but unlikely, that installing the short/small AC belt has pulled the crank or balancer seal in a manner that is creating a loss of suction at the pump.
If the dash instrument still does not respond, it may be helpful to temporarily install a mechanical gauge to the test port near the rear of the intake and measure actual pressure.