I made a new Oblivion character and i want the time of this character's life to be set after the events of Oblivion so the time Skyrim takes place. anyway i'm working on his backstory. he was hired to go investigate the Arena as the one who hired him thinks it's fixed. so is the Arena still open? do the Thalmor control it?

It should still be active. There's mention of the Grand Champion of the Arena getting assassinated while taking a stroll in the Great Forest in 4E 187, just 13 years before Skyrim takes place.

Possibly, although I always pictured the Imperial city to be in pretty rough shape after the Great War. Or at least worst off then when we saw it last saw it in Oblivion.

Where is that mentioned?

Cicero's Journal.

Ah. never read those.

In times of civil unrest, I image arenas are more popular than ever.