Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the rolled oats, chocolate chips (or raisins), and chopped nuts (if using).
Using a tablespoon, drop heaping mounds of cookie dough onto the prepared baking sheet, leaving some space between each cookie as they will spread.
Bake in the preheated oven for 10-12 minutes or until the edges are golden brown. The centers may look slightly underbaked; they will continue to firm up as they cool.
Remove from the oven and let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.
Notes
You can substitute chocolate chips with raisins for a different flavor.