There are 2 o-rings between the block to oil pan (to oil filter) and it's not unusual for one of those o-rings to distort. The pan must be removed. I'd also replace the oil sump o-ring (remove sump).
The oil pump is internal and driven by the crankshaft. There is an oil pump o-ring, gasket and front crank seal that can leak too but that's usually not a cause for low oil pressure. When those seals leak, they leak externally and you'ld see oil leaking from the front (right side, crank pulley) area.
To remove the oil pump, you need to remove the t-belt and crank pulley, use a puller to get the lower cog gear off and remove the pump assembly.
I'd start by removing the oil pan as it's easier, cheaper and will probably solve your problem.