check oil light, frozen dashboard computer and engine knocking

What is your question?

1. Every time I start my car, the yellow oil light comes on for about 15 seconds and then shuts off. I check my oil level almost daily, but the level is not low. What's this about? 2. When I went to change the clock in the car for daylight savings, I tried to bring up the clock display on the dash with the button on the directional stick, but it won't change from the outside temperature. I don't know if it's frozen and needs to be reset or what...can you help? 3. My service engine light is on and my mechanic changed the air flow sensor, but the light came back on. I went back and he said the two from oxygen sensors need to be replaced. First, is it okay for me to drive with this issue, as money is really tight. Second, I notice that in 2nd and especially 3rd gear, when I give it gas, the engine starts knocking, even more so going up a steep hill. I always use premium gas. I don't know what the problem is...any suggestions?