The buttons are part of the second arg to the messagebox function which contains 3 components.

numbuttype+numicon+numdefaultbut

Value Dialog box buttons
0 OK button only
1 OK and Cancel buttons
2 Abort, Retry, and Ignore buttons
3 Yes, No, and Cancel buttons
4 Yes and No buttons
5 Retry and Cancel buttons

Value Icon
16 Stop sign
32 Question mark
48 Exclamation point
64 Information (i) icon

Value Default button
0 First button
256 Second button
512 Third button

So, OK only as a question would be 0+32+0