But if you press the first Button, the oleTest runs well.
Sure, the way you have MainLoop; before oleTest(), the first button destroys the Tk event loop, leaving OLE to run unfettered.
Apparently the Tk eventloop and the way OLE runs, are not compatible, but I can't test that here. Usually, you would put your oleTest() into a separate thread, if it interferes with Tk's eventloop.