>In the existing application button \<OK closes a form. Shortcut ALT+O was meant to be used to trigger this button click event. It turned out that just pressing single O/o on a keyboard also triggers this button click event and closes the form. I created a form with a single button \<OK and it is the case. Is it known behaviour and if so, is there way without changing existing caption \<OK to avoid closing a form on pressing just 'O'.

Pretty certain it could be done, but as this goes against common expectations you would puzzle new users AND maintainers so I'd argue against it.