In all probability the OK button will take you into a dialog where you can accept or reject something, or act as a permanent dismissal of the thing you're seeing if it acts as the acknowledgement that you've seen it (and that's often the case for announcements - where the Not Now button serves as a "show me this again later" option).
But not knowing precisely what you're dealing with here I cannot state the above with certainty. A screen shot or knowing the actual message text of the thing would help immensely in identifying what it is and why it's happening.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Tolerance is the positive and cordial effort to understand another's beliefs, practices, and habits without necessarily sharing or accepting them.
~ Joshua Liebman