This planet's atmosphere has oxygen at a partial pressure that is breathable to humans, and the rest of the atmosphere is, at least, not made up primarily of Nitrogen (that is, it's not just Earth's atmosphere with marginally different partial pressures). The atmosphere must be breathable for at least long enough to mount a rescue mission (say a few days without any ill effects, and a few months before death), although I'm also happy for it to be breathable indefinitely.
The atmosphere must be made of gases that are reasonable to exist in the quantities required. For example, it's unlikely to be made of Xenon gas, since that's fairly rare (as far as I know), even though Xenon is probably breathable.
I am aware that, at high enough pressures and low enough partial pressures of oxygen, some really exotic gases are breathable. I'm good with that, although one final stipulation is that the atmosphere cannot be so dense that gas narcosis sets in (I just really don't want to deal with that in my story is all).
So, what's my atmosphere made of?