Check that your oil filter is not blocked, the tube up the centre of the filter housing should have a small hole in it about an inch from the top. You may have dislodged some crud whilst the motor was apart that has passed through the filter and blocked the hole. Easy way to tell is to bypass/disconnect the filter and see if it makes a difference to your oil pressure.
Also if your oil pressure light is not glowing then your pump should be making the required pressure (30 -40psi?) so it would not be your pump.
This assumes you are running a grey motor.