Oil leak

That is a real common symptom of someone forgot to replace the "oil" cap.

The other cause is oil is leaking onto someplace where it can pool, then at high engine speed the increased wind blows it out over the engine. Look for any wet areas on the engine, then start by looking for the highest place you see that oil, keeping in mind oil runs down, but it can be sprayed up. If the source of the leak is not obvious, wash the area with engine degreaser, then add a small bottle of dark purple dye to the oil. Check a day later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores will have the correct dye for the fluid being tested, and those that borrow or rent tools should have a black light.