The main one is Walcott, he is our big game player. Ramsey was ridiculous and would've helped us beat a team like Swansea or Stoke. Ozil has been terrible and a break from the game is good for him I guess.
In the end we don't deserve to win the title as we don't have a squad capable of dealing with such injuries and we have an awful striker. I guess it's something that always happens to us, you don't see Man City losing Toure or Liverpool losing Suarez for as long as we've missed guys like Walcott and Ramsey. With a bit of luck and those guys staying fit, then yes, we would be in the mix as those guys easily make a lot more than 6 points difference.
Oxlade-Chamberlain was injured. He started the first game of the season and would have played a lot had he not got injured. The guys who have benefited from the injuries are Cazorla and Podolski, though Ox too to a lesser extent.