I have some confusion about the way a function "processOK" is being called here. it usually is something like command=processOK(). I ve recently started learning Python as an additional skill, would be thankful if you could clarify. Also please state the difference between a normal class and widget classes.
btOK = Button(window, text = "OK", fg = "red", command = processOK)
The only significant difference between a widget class and a "normal" class -- whatever that is -- is that Tkinter widget classes wrap objects created by the underlying tkinter engine (specifically, an embedded tcl/tk interpreter).
As for the command options of buttons, it takes a reference to a callable function. So, it would be ..., command=processOK, not ..., command=processOK(). If you need to pass arguments to the function you must use lambda, functools.partial, or something similar.
