I have a relatively successful app in the app store that allows people to view metrics on their iDevice using a JSON file hosted on the their server. The app has a simple settings screen in which you simply type the URL of your JSON file and the app takes care of visualising the data in the file. I use AFNetworking for this.
For example the URL might be: <http://www.mylargecompany.com/factorykpi.json>

Customers are now coming back to me and asking for the ability to connect to their servers more securely. Problem is there is a myriad of ways you can secure your server.

I need some advice. What kind of standard security features would I need to build into my app. I am confused by OAuth, HTTPS, etc. 

I believe OAuth would mean the customers server would have to use it? Can you make a generic OAuth login screen in an app or do you need to know which web server you are connecting to authenticate.

Any advice on even the most basic of security measure would be very welcome!

Regards, 
MonkeyBusiness