Yea... please be a bit more specific. Under what circumstances does it come on? if its around corners and/or under hard breaking, its just low on oil. My cars all do that if I forget to check the level periodically.

If it seems totally random, check thoroughly that the wire from the sending unit hasn't worn through or somehow can ground out to the engine or chassis such as through the firewall, or pinched between the head and valve cover, burned on the exhaust manifold... I've seen all of these circumstances many times.

Unless the motor was built improperly, a B18... or about any Volvo motor, should never have oil pressure problems. They generally wear out elsewhere first.

Does the oil light take longer than normal to go out after first startup? Any quiet knocking when you first start? Or when that light does come on?

My guess is on the wire or the sending unit itself. But you best get it figured out before you drive it too much, as the damage caused by no oil pressure is disastrous.
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-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC