Preheat oven to 375 degrees. Line a baking sheet with parchment paper and brush with olive oil.
Grate 2 of the zucchini so that you have 2 cups. Add to a large bowl, and mix with the oat flour, baking soda, salt, lemon juice, oregano, and egg.
In a small food processor, pulse garlic and walnuts together until a coarse meal is formed. Add olives and onion, and pulse again until mixture is sticky. Empty into the bowl with the zucchini, and mix well.
Gently stir in the feta. The mixture should be sticky, but workable. Roll into 12 1-inch balls, adding flour if necessary so they come together. Arrange on the baking sheet and brush the tops with olive oil.
Bake for around 30 minutes, until the bottoms are golden brown.
Using a vegetable peeler, peel long strip of the zucchini, making wide noodles. Serve greekballs over zucchini with sliced tomatoes, crumbled feta, a drizzle of olive oil, and freshly ground pepper.
Recipe by Veggie Ninjas at https://www.veggieninjas.com/zucchini-greekballs/