Nate.....
02 Sensors won't cause hard starting issues. You either have a spark issue(not likely) or a fuel issue(sounds like your symptoms), or you're not getting enough air, or too much air to the engine.
You need to let us know what the symptoms are.
Also.....a real common problem is the Idle Air Control Valve failing. This controls the idle of the car and if it is stuck, the car will hard start.
Try giving the car a slight amount of gas(push down the gas pedal VERY slightly) when trying to start it, this will eccentially be using your foot as the control for the idle. Give it NO MORE than 1/4 throttle, and if it doesn't work right away, than its not the problem.
Like JP said....check fuel pressure...there are two ways that you NEED to do this.
First, have someone kneel on the passenger side of the car by the rear tire and listen for the fuel pump. When you turn the key ON(lights on the dash come on) the fuel pump will "prime" for 3 seconds, have that person listen for this. The pump should be a steady buzz, and not a crunchy or intermittant sound.
Second, after you "prime" the fuel pump, you need to go to the little schradder valve on the engine. Its located on the fuel rail by the oil cap and has a little plastic cap on it. Its best described as looking like a tire valve stem used to fill air in a tire.
Remove this cap, and with a small screw driver and a rag in hand, depress the little valve inside and see how much fuel squirts out. BE SURE NOT to have your face/eyes by this as it may spray out.......use common sense. Also do NOT try this on a hot engine.
If it squirts a good amount out we'll move onto the next step. If there is only a drizzle, than you may have a bad fuel pump or something of that sort.
Let us know what happends......oh, don't waste too much time trying to get the codes, they probably won't tell you whats wrong because of the fact that the problem is when you're starting.
Ditto on changing the fuel filter. The plugs WILL cause starting problems, but only over time, not something that will just stop working one day, so wait on this.