Chevy 4.3 intermittent start?

The symptom of a bad O2 sensor is a trouble code. It will not affect anything you can determine except fuel mileage. It would be helpful if you told us what you have to do to get it started when it has a bad time starting. Does it just start finally? Does it "click" when you try to start it? Does it make a whining noise when you try? Do you have to jump it off? Push it off? Whack the starter? Short out the solenoid? Loosen the starter and do it again? What?

In that case is it getting fire to the plugs? Does squirting a little fuel in the intake start it?

You will have to try some things. First thing would be to squirt some starting fluid or a little fuel into the intake. If it doesn't hit then it is the ignition. You could have a bad ignition switch(the electrical one) or something like that. What kind of ignition does it have? A single coil with current to it would be the easiest to check. See if current is coming to the coil from the ignition switch and also from the starter (I) post when trying to crank it.

Weak ignition coil or fuel injectors sticking is common on these,there is an update version to prevent this is to replace the fuel injector spider assembly.

Check the wires going to starter, check relay, check fuses/circuit breaker. Check those first. Check the starter under load. Check to see if your getting fuel to the injectors.

I have encountered this problem before. It was due to " Bad spots " on the starter . The 4.3 V6 is notorious for this issue.

try new fuel filter