In a large bowl, cream together butter, white sugar, and brown sugar, until light and fluffy. Beat in the eggs one at a time until well blended, and then add the vanilla extract.
Combine the oats, flour, baking soda, and salt. Add into the butter mixture and beat until just combined.
Fold in chocolate chunks and hazelnuts. Chill cookie dough, covered, for at least one hour or up to three days.
Preheat oven to 350°F. Shape dough into walnut sized balls, and fill each cookie dough ball with about 1 teaspoon of Nutella. You can do this by flattening the dough and pressing it around the Nutella, or using a cookie scoop, make an indentation in the middle of the dough, place the Nutella in the indentation, and then cover with more cookie dough. Form the dough into balls and arrange about 3 inches apart on baking sheets lined with parchment paper.
Sprinkle cookies with flaky sea salt, if desired. Bake cookies for 12 minutes, or until just pale golden brown. Cool cookies on baking sheet for 5 minutes and transfer to racks to cool completely.