Don't keep turning the engine over if it won't start - the catalyst will fill up with unburnt fuel and will cause damage (especially when it finally does start and melt the cat!).
I'm not at all familiar with the Explorer but there are several things you can check yourself.
1. Make sure the vehicle has plenty of fuel and that the fuel filter isn't blocked and the pump is working correctly.
2. Ensure ignition circuit is okay, check that the LT leads are secure on the coil. On some vehicles they have special distributor rotor arms (fitted with resistor) that can break down slowly causing misfire/awkward starts (a very common fault these days!). Whilst distributor cap is off check that all contacts are okay, especially the centre coil contact. Check the fusebox and that all ignition/fuel/ECM fuses are not blown.
3. Inlet manifold - make sure that all sensors and vacuum pipes are secure. Is there an inlet pipe between air filter box and manifold that you can remove to have a look at the throttle butterfly and air flow sensor? Some crap could have passed thru the air filter and be obstructing a sensor.
4. What sort of auto-choke mechanism is it on board? Again, check everything attached to the throttle housing.
5. Does it have an Electronic Control Module? Make sure connections are secure and on some they have a vacuum pipe as well. Make sure all pipes are on not constricted.
6. Is there a fault diagnosis module built into the car? Some cars have this either around the drivers fusebox area, passenger footwell or in the boot - check your car handbook or phone a dealer and ask if it has one fitted. If there is a fuel/ignition fault it will display a fault code via a flashing LED. Again, being unfamiliar with the Explorer you may need a dealer to use a special fault diagnosis machine to check certain components.
7. Check the cambelt tension, if it's very loose it may have jumped a tooth and changed the timing, or caused valve damage.
8. Do you have a friend with a similar vehicle that you can try swapping the O2 sensor with?
9. If you can't see anything wrong at all then consult a garage who can inspect the vehicle - it's hard to explain but you can often hear what the fault is and know which system to illiminate.
Hope some of the above can be of use, I stopped being a mechanic back in 1996 but these are things I'd often look for on a non-start.
Good Luck!!
