Best, average, and worst case (and any other mathematical function from an ordered set to another ordered set) can all be described with big omega, theta, and O notation. The two concepts are orthogonal.

For example:

Quicksort's average-case time complexity is Omega(1), also Omega(n log(n)), Theta(n log(n)), O(n log(n)), and also O(e^n).

Its worst-case time complexity is Omega(1), also Omega(n^2), Theta(n^2), O(n^2), and also O(e^n).