Control buttons
Pressing the "Cancel" button will cancel OldMovie and return to your graphics application without any changes to the image.
The "OK" button will start rendering the effects currently configured in OldMovie to the image and return to your graphics application. Depending on your graphics application, a progress meter of some sort will be displayed to indicate the time estimated to finishing rendering the effects.

Help button
As you've probably noticed already, the button with the question mark will open the on-line help system, which you are reading now.