So in many space fighting games, if the game is 1st POV, there's aliens. And, it's common that the humans are fighting on alien ships. BUT, the weapons are often conventional (no plasma cannons) weapons with fire/recoil/stuff that regular weapons do on Earth. These (Earth-based) regular weapons use explosions, which use air. But these (Still Earth-based and regular) explosive weapons are on alien ships, which doesn't have probably don't have oxygen. Which means that the weapons can't fire. Are there any kinds of gun/weapon (existing or not, I know the rpg) modern weapons in space combat, without oxygen? I'm assuming there's no oxygen on the alien ship for weapons because it's highly likely that aliens don't breath oxygen. And to make it more specific, weapons that you suggest MUST be possible. I need weapons that are well designed to work in zero gravity, and zero air. I also am assuming that the people who fire the weapons need to shoot multiple times and can't have much recoil. For example, the RPG launcher is a pretty good weapons but it's a waste of energy against humans. So effectively an anti-personal zero-recoil weapon is what I want.
Self-oxidising propellant for bullets is already a thing, that's what saltpetre does, it oxidises gunpowder. As long as the bullets are well sealed against atmospheric intrusion from exotic gases, which they are almost by definition then you have no problems with using any modern firearm, or any weapon based on modern firearms technology on an alien spacecraft. Recoil is more of an issue in microgravity but that's the only hassle and that's a problem of some magnitude with any projectile weapon regardless of what said projectile is.
The recoilless rifle is also already a thing, as is the floating barrel, both of these designs drastically reduce the apparent recoil felt by the user. In combination a floating barrel with adequate gas venting eliminates the issue altogether.
I need weapons that are well designed to work in zero gravity, and zero air. I also am assuming that the people who fire the weapons need to shoot multiple times and can't have much recoil.
Firstly, lets think about the issue of recoil. This physics.SE answer gives you an idea of the order of magnitude here... you won't go flying back like you're holding a rocket, and you won't pinwheel around like a firework.
Next, if you ever look at footage of real soldiers in action, what you see is lots of shooting from behind cover, weapon braced. It makes aiming a little easier, soaks up some of the recoil too. What you don't see is classic FPS running and shooting, or (heaven forfend), circle strafing.
Your scifi marines (or espatiers, if you prefer) will not just fly in like little jets spitting out bullets, because they don't want to end up as skeet. They're going to advance carefully, they're going to brace against walls, they're going to use grab points or magnetic fixtures or whatever. That recoil is controllable, and control it they will.