When the oil pressure is low or 0 at idle, try to measure the oil on the dip stick. My experience with a 2000 V70 was, all the oil being pumped up to the upper engine and the return ports were too clogged to let it return to the pan. So the pump was sucking air and light would come on. I know this sounds bazaar but it was true. Half of the oil passages in the head were clogged, oil return for the turbo was clogged (pushing oil into the exhaust and making a huge cloud of white smoke).
This engine sits in my garage after replacing it with one from a wreck.

I originally thought my problem was the "O" rings but replacing them did no good.
My oil pressure light would com e on while driving. Stopped and checked the oil and it was full. Never thought to check it when the light was on though.
The car was an Ebay purchased southern car. Didn't check it out thoroughly, but now I believe the oil was never changed in it's first 50K miles of ownership and never driven on trips longer than shopping and going to church.
Why did you rebuild (only) the lower end?
Good luck
Tom