I am not familiar with iOS programming but it seems your end point is invalid. As a result you will get "page not found" HTML response.

Instead of using `http://stackexchange.com/oauth/2.2/users?order=desc&sort=reputation&site=stackoverflow&Client_Id=5029`, try to use `https://api.stackexchange.com/2.2/users?order=desc&sort=reputation&site=stackoverflow&Client_Id=5029` and you will get the list of users.