Dear Syed Taimoor Ahmed,

I think, NO. Because big-theta is a combination of big-o and big-omega. Big-o is used to show upper bound of the complexity of the algo. But  big-theta is used to show average case of complexity of the algo. It contain both upper and lower bound. 

So.

Big-theta(nlog n)=   

C1*n log n

Big-o just give you an upper bound on the complexity while Big-theta give you the "exact" complexity.

For example, an algorithm that is linear on the size of its input is O(n^2) since n

http://stackoverflow.com/questions/2307283/what-does-olog-n-mean-exactly

http://stackoverflow.com/questions/2095395/is-logn-%CE%98n-logn