Preheat oven to 350°F/180°C/Gas 4.
Line sheet pan with parchment paper.
In a large bowl, beat butter, brown sugar and granulated sugar until creamy.
Add eggs one at a time, mixing after each. Add vanilla and mix until well combined.
In medium bowl, combine flour, cinnamon and oats. Add to butter mixture a little between each mix. Fold until combined.
Drop by rounded tablespoons onto prepared pan, spacing about 2 inches apart.
Bake until edges just begin to turn golden, about 9-11 minutes.
Let cool on pan for 5 minutes, then transfer to wire rack to cool completely.