check engine oil light on/off

I have a 12 years old Toyota Camery . It's about 150K Mileage. It's my daily commute car and it still runs great. I just noticed last week that the ' check engine oil" indicator light started on and off for quite a lot. I replaced the engine oil and the oil filter this weekend. However, the " check engine oil" indicator light still on and off from time to time. Any clues ?

Thanks.