Namaste is used frequently, both formally and informally. If talking to elders, seniors you can add the word ji, to make it more respectful as in "Namaste Ji". In the metros or larger cities, younger generation who feel inglicized (Engli-cized) may use the english words Hi, hello, hey, wussup, sup etc to greet each other but majority of manland hindi speaking people will still use namaste. If you happen to be In Punjab, we use Sat Sri Akal as well
सत श्री अकाल is a Sikh greeting that roughly translates to "the truth is eternal." It is used when both meeting and parting, like नमस्ते. Reference: Bhardwaj, Mangat, and Gordon Wells. Hindi Urdu Bol Chaal. A Beginner's Course In Spoken Hindi And Urdu On BBC Television, Course Writers: Mangat Bhardwaj, Gordon Wells. London: BBC, 1989. Print.
A native Urdu speaker here, learning Hindi script. There are two issues with this short phrase. One: Namaste is being thrown around all over the place, it can be used for hello, bye and even as a formal greeting replacing Salaam in Urdu. But the second part , ठीक है, in literal translation would be "It is alright/fine". The word Okay, OK, even in English has a rather complex etymology and I think, it should NOT be used as the translation at all. Here is a link to the origin of word Okay
In other words, even in English the word Okay is rather an abberation and the origin of the word has many possibilities.