Preheat oven to 375 degrees. Have a 9 inch springform pan ready. Using a blender or food processor, blend the oat grahams until they are a crumb consistency. Place the graham crumbs in a large bowl. Add the sugar and melted butter. Mix together until everything is incorporated.
Pour into springform pan and evenly press crumb mixture firmly on the bottom of pan up the sides about an inch. Bake for 7 minutes and then remove from oven. Allow to cool for 30 minutes.
While the crust is cooling, zest the key limes to get about one tablespoon of zest. Juice the limes to get just a bit more than 3/4 cup of lime juice. Set aside.
Preheat oven to 350 degrees. I like to wrap the bottom of the springform pan with aluminum foil, just in case any filling seeps through. I also place springform pan on baking sheet.
In a large mixing bowl, whisk together the sweetened condensed milk, sour cream, lime juice, and zest.
Pour into prepared graham cracker crust and bake for 10 minutes.
Let pie cool slightly before chilling. Chill for at least 3 hours. Top with whipped cream and lime slices, if desired.