Yes. OK saves the current tag data back to the file.
MP3tag does not compare the current tag data with a possible previous state.
See e.g. this thread:
Currently, pressing Ctrl+S saves the tag entered in the Tag Panel to all selected files, regardless of whether they require changes or not.
My suggestion is to add a additional Save (minimal) entry in 'File' menu. (with keyboard shortcut, e.g. Ctrl+Shift+S).
When pressed, it would save the tag only to the affected files, skipping those that do not require modification. This would speed up the process if there are such files in the selection.
In such a case, the status bar at the bottom of …