SAS has a very nice gym located in their domestic lounge at OSL when I visited and they have clean shower rooms with lockers as well. This was one of the best I have seen and probably the first one as well. I never thought of it as necessary since chances are low that most transit passengers would want to have a workout during their layover. The hassle of changing and showering after the workout might mean it is rarely on travellers' mind. Below is a photo of the gym with its facilities.
