Preheat oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper, leaving an overhang for easy removal.

Prepare the crust: Mix graham cracker crumbs, 1/4 cup sugar, and melted butter until combined. Press firmly into the bottom of the prepared pan and bake for 10 minutes. Set aside to cool slightly.

Prepare the rhubarb filling: In a medium bowl, toss the chopped rhubarb with 1/2 cup sugar, cornstarch, and lemon juice. Set aside.

Prepare the cheesecake layer: Beat the softened cream cheese and 1/2 cup sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla extract. Pour the cream cheese mixture evenly over the slightly cooled crust.

Layer and top: Gently spoon the rhubarb mixture over the cheesecake layer. In a separate bowl, combine all oat crumble ingredients until a coarse, crumbly texture forms. Sprinkle the oat crumble evenly over the rhubarb.

Bake for 45 to 55 minutes, or until the topping is golden brown and the cheesecake is mostly set (a slight jiggle in the center is fine). Cool completely on a wire rack, then chill in the refrigerator for at least 4 hours before slicing into squares.