The oxmathproblems.cls was written by Keith A. Gillow. The latest version (1.3) was released on 12/5/2020. The class is based upon the standard exam class.
For more information please see the corresponding problem sheet class FAQ page
A simple trick to decorate Theorem-like environments with poker suits QED symbols.
I did not come up with this theorem decoration style (I've first seen it here) nor with the whole code (I salvaged it from TeX StackExchange and other sources over the years). This is just my current implementation of the code.