It looks like we missed each other and I am leaving for the night, but I can summarize your possibilities for when you return if I am not here.
What you first need to have the shop (or yourself if you are able) do is first unplug the oil pressure switch and see if the light goes out. The oil pressure switch is the one wire switch that is screwed in at the front bottom of the engine by the oil filter (accessed from below).
If the light stays on even with the switch unplugged, then you know you have a wiring issue. This is because the light turns on when the oil pressure switch has no pressure and grounds... if you unplug the switch there is no way for the wire to ground, so if the light is still on then you know the wiring is grounded somewhere.
If the light goes out when you unplug the switch then you know there is either a physical low oil pressure issue or a bad switch. You have two options at this point, you can either have the oil pressure tested with a mechanical gauge (installed in place of the switch temporarily), or you can just replace the switch to rule it out. Most people opt to replace the switch due to the cost; having a mechanical gauge test performed is usually an hour or so labor, around $100, whereas by comparison the switch is usually about $25 and take a few minutes to change, so it is cheaper to just try it than test it. The upside to having the pressure tested of course is you get a better idea of the exact pressure, whereas if the switch is not the issue you only know that there is low pressure, not if there is zero pressure, near zero, near threshold, etc.
If you determine it is a pressure problem, the prognosis worsens drastically. The pressure loss is primarily going to be bearing wear in the engine or oil pump/regulator failure. On extremely rare occasions we'll see certain brand oil filters cause issues as well, though it is usually not consistently "on" like you describe.
If I understood wrong and the light is not on pretty much all the time, then you could try a fresh Mitsubishi oil filter and new oil first (even slightly heavier oil), but if it is on all the time (not just at low idle, flickering etc) it is unlikely to help. Rather the pressure issue is going to need to be looked into, pulling the oil pan to check bearing clearances, remove the regulator spring to check for damage from the pump etc. If it goes to this point, I would recommend getting it to a shop with more internal experience at that point (the one it was at probably isn't the best choice for something like this if they were uncertain about the light being on etc).