1999 Honda Civic DX O2 SENSOR HELP!

3 quarts low and you ask about compression. First of all a car is nice to drive but they do have to have someone watch out for them. The oil system is a 4 quart system and it was headed towards dry. YOU have to check the oil. You are positive that it doesn't have a leak. How can you be so confident and what even caused you to check the oil to find it low? How many miles on the oil change? Is there a fog behind you when you drive?

The O2 sensor can wait. It will not kill the car but the problem with the oil has to be rectified. It might even have pretty good compression. Nail down where the oil is going first.

The plain truth is, it used to be a standard thing to check the oil at the gas station. You could even sit in your car and have the guy do it for free. But it got checked often. Today, we don't want to look like there could be something wrong with our car so we grab gas and drink and head down the road. After that we forget about the car and drive until it lets us know it some way that it is having problems. My comment is that you have missed checking it and the signs it was giving you.