Process the oreos and salt together in a food processor until you get cookie crumbs.
Then drizzle the butter in while running the food processor at its lowest setting (this makes the cookie crumbs moist enough to be pressed into a cheesecake base).
Butter the insides of 4 egg rings or tart rings. Place these on a parchment paper lined cookie sheet or tray. (Alternatively, you can use 4 mini fluted tart pans as well).
Place some of the cookie crumbs inside and press down the crust to form a flat base using a flat bottomed glass, or tart tamper, or the back of a spoon.
If you'd like to have a crust riding up the sides of the cheesecake as well, you can use your fingers and press the cookie crumbs onto the sides of your egg rings/tart rings (this is optional - just the base would be sufficient and easier).
Freeze the tart bases for a few hours, or until you need them.
Carefully remove the tart ring by using a sharp knife to loosen the edges. Be careful not to break the bases. If you have molded the sides of the cheesecake as well, you may need a blow torch to warm up the sides slightly, to make unmolding easier.