The Products tab lists all products that are associated with the given option set. You can remove products associated with the option set from the list view here.
Click the Select products button to open the Add products window. This will list the first 200 products in your store. Enter a search term to find more products not listed initially.
Options already associated with the option set will show a green pill on the right side. Options associated with other option sets will show a grey pill. Selecting a product already associated with an option set will change the option set that product is associated with.
A product can only be associated with a single option set.