Didn't see anyone using oxygen to get to the summit of Kilimanjaro but there were people who were treated with it while coming back down. Funny thing is that the guides generally didn't have canisters on them, they had latex gloves inflated with oxygen and would let people who were delirious to take a hit off the glove.
I used Diamox, starting with half a pill on day two, moving to a full pill on day four. I didn't have any problems with exhaustion but did have some mild hallucinations starting around the time we had lunch at Karanga.
As far as condition goes, I did a good bit of hiking on local hilly terrain but didn't do anything special beyond that. 4000 ft was the max altitude for that hiking so it did nothing to help prep for altitude issues. The other big difference is that Kili has areas of lots of scree, which can be annoying if you're not accustomed to it.
A year later I hiked around Ecuador and had some issues with getting out of breath at high altitude but I wasn't taking Diamox and we often drove to a high starting point so acclimation was minimal.
More than anything, paying for more days on the mountain is what will have the biggest impact on your odds of success. If you can find a copy of the Imax movie about Kilimanjaro, you'll find the climb more interesting as you'll have background info on many of the things you'll see.
