Add basil, pistachios, and garlic to food processor and pulse until combined.
Add Parmesan and pulse a few times more.
Scrape sides of food processor bowl down.
While the food processor is running, add olive oil to bowl in a fine stream until desired consistency is achieved.
Notes
The 1/4 cup of olive oil is a great starting point. It just depends on what consistency you are looking for. I have added as much as 3/4 cup to get a thin sauce for their pasta.