The oil separator is a system that maintains and regulates crankcase ventilation and pressure. If the car is running lean that could be an issue. As well the O2 sensors aret very expensive and when they fail they usually cause a check whine warning light.
One thing you can look into is having the OBD scanned for Codes. The Code it throws will give you a good indication of what is wrong. Some auto parts stores like Advance carry OBD scanners and will scan it for free.