The function OSUser() only fetches the userID of the person that is actually accessing the app. If you want to translate that into an actual user name, you would need to get that info from your Active Directory, or other User Directory. Even then, I'm not 100% sure how you get the actual user name. You would need to create some expression that calculates the user name based on the UserID.
Can you give us some information on what the requirements are for this? Do you need to create a list of usernames? Are you just "welcoming" that user by displaying a message like, "Welcome user T569382", and want to be "Welcome John Wick"?
Also, do you have access to the proper directory to fetch the User Name associated with a UserID?