Like Valleytech said, try to see where it's coming from. These engines generally leak from a few places, oil cooler, rear main seal an the crankcase breather housing to name a few.
Usually its the valve cover or the oil drain plug and start by putting a piece of card board under the engine area and that will help zero in on the location
RepairPal Certified shops can diagnose and repair your 1998 Volvo S90. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.