Gather and prep all ingredients.

Heat olive oil in a skillet over medium-high heat. Add mushrooms to the hot oil, sprinkle with salt, and cook and stir until the liquid from mushrooms has evaporated. Stir butter into mushrooms, reduce heat to medium, and cook and stir mushrooms until golden brown, about 5 minutes.

Stir onion into mushrooms and cook, stirring often, until onion is translucent, about 5 minutes. Remove skillet from heat and stir garlic into the mushroom mixture until fragrant, about 1 minute. Transfer mixture to a mixing bowl.

Mix oats into the mushroom mixture until thoroughly combined. Gently stir 1 ounce Parmigiano-Reggiano cheese into the mixture. Add breadcrumbs, 1/4 cup parsley, and 1 egg; season with salt, black pepper, cayenne pepper, and oregano. Mix together with a fork until crumbly. Stir in remaining 1 egg. The mixture should hold together when pressed.

Cover the bowl with plastic wrap and refrigerate for at least 4 hours. For best flavor and texture, refrigerate overnight.

Preheat the oven to 450°F (230°C). Line a baking sheet with a silicone baking mat or parchment paper.

Form the mixture into small meatballs using a 2-tablespoon scoop. Roll meatballs lightly between your hands until smooth, if desired. Arrange meatballs on the prepared baking sheet.

Bake in the preheated oven until meatballs are lightly golden brown, about 12 to 15 minutes.

Bring pasta sauce to a boil in a large saucepan; reduce heat to low. Gently stir meatballs into the sauce until coated. Simmer meatballs in the sauce until cooked through, 45 minutes to 1 hour.