Assuming you meant oil pump, it's what your oil filter screws onto. Buick 3.8 V-6's have an external oil pump. That's good news if that's your actual problem. If it's bearings or a plugged or broken pick up tube, things get a little rougher. Good luck.
RepairPal Certified shops can diagnose and repair your 1999 Buick LeSabre. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.