Adjustments should be made depending on the filling you're using. If your filling is on the sweeter side, consider reducing the sugar a bit, otherwise keep it as is. Being someone who likes things a little less sweet, I obviously lean towards the former.
Adapted from All Recipes3/4 cup old fashioned oats
1/2 cup all-purpose flour (65g)
1/2 tsp cinnamon
1/2 to 2/3 cup packed golden yellow/light brown sugar (110g to 147g) -- depending on how sweet your filling is
6 tbsp unsalted butter, cold (84g)
In a medium bowl add oats, flour, cinnamon and sugar and whisk until combined.
Add butter and using a pastry blender or two forks, cut the butter into the dry mix until crumbly.
Top pie or crisp with crumb topping and bake per recipe instructions.