In a large mixing bowl, combine the rolled oats, peanut butter, honey (or maple syrup), and vanilla extract. Mix until it's well combined and sticky.
Add mini chocolate chips and chopped nuts to the mixture. Fold them gently into the oat mixture until evenly distributed.
Line a muffin tin with paper liners or spray generously with cooking spray.
Scoop a generous tablespoon of the oat mixture into each muffin cup and press down firmly with the back of a spoon or your fingers until they are compact.
Once all the mixture is in the cups, place the muffin tin in the refrigerator to set for at least 1-2 hours until firm.
After they have firmed up, remove from the muffin tin and peel off the liners if using. Store any leftovers in an airtight container in the fridge.
Notes
Serve the oat cups on a colorful platter; you can drizzle some extra peanut butter on top and sprinkle a few chocolate chips for a visual treat!