Özil is not a midfielder. He's a forward in my book.
Ramsey's a walking one-man midfield basically, even on an off day. Without him there would be no connection between the team parts. I think innervisionscm put it pretty well above. Ramsey gets involved in everything, and so he loses out a few times. But the massive amont of good work he does both on and off the ball simply can't be ignored. He's not in the team to be a tiki-taka player. He's playing because he's everywhere at once. You take a closer look who's covering defensively when Coquelin's caught snoozing and it's Ramsey who's been tracking Bolasie across the whole field. Someone almost scores at the near post a couple minutes later with a cheeky backheel flick from a superb Özil cross - that's Ramsey again.
Wenger said once he knew he had confirmation that Pires was good because Bergkamp started looking for him in every training session. Great players seek out other great players. It's pretty telling that Özil has such a great understanding with Ramsey with that in mind. It's not Alexis, it's not Giroud, it's not Wilshere (though there's hope for that one yet, if Jack ever stays fit) and it's not Walcott. It's Ramsey every time.