In a bowl, combine the pears and cinnamon and toss to combine. Toss in the lemon juice.
In a separate medium bowl, whisk together the oats, flour, brown sugar, and salt.
Add the butter and use your fingers to work the butter into the flour until the mixture comes together and the crumbles are about the size of peas. Mix in the pistachios and set aside.
Pour the pears into a greased 8-inch baking dish and cover with the oat mixture.
Bake until the top is brown and the pears are bubbly, 40 to 45 minutes.