The defence was fine, not amazing but not bad either. Bellerin didnt play in the league when Ozil arrived, he played in 14/15, so you have your facts wrong, again.
Our defence in 2013/14 when we signed Ozil was:
Shezza
Sagna - Merts - Kos - Monreal/Gibbs
Like I said, not amazing but not bad either. Not that I rated him but we lost Song in 2012/13 so Arteta was the defacto DM when Ozil joined and Wenger brought back a washed up Flamini. As much as I hated Song he was a better DM than Arteta! We endured that for a season then Arteta got limited playing time and Coquelin shared DM duties with Flamini.
In 2015/16 Wenger added Elneny so we had 3 shit DMs (4 if you include Arteta) that all got Prem game time that season, sharing the DM role!! In 2016/17 Wenger compounded all of his DM mistakes by bring in Xhaka, so now we have him, Coquelin and Elneny all getting Prem game time as a DM.
This is what I mean when I say Wenger did it half-arsed!! You can't criticise a perfectly adequate defence when you play gung-ho football like Wenger did but you don't have a single decent defensive screen in the midfield to help ! Ozil always had that screen at other clubs, thats why he was able to perform so well.
At Real he had L Diarra, Alonso and Khedira as defensive shields and Khedira for Germany