Heat the oven to 375°F. Line a 12-cup muffin pan with paper liners or grease the cups lightly.
Mash the bananas in a large bowl until mostly smooth, about 1 1/4 cups. Whisk in the eggs, Greek yogurt, maple syrup and vanilla until evenly combined.
In a second bowl, stir together the flour, oats, baking powder, cinnamon and salt. Add to the banana mixture and fold until just combined, then fold in the walnuts.
Divide the batter among the 12 cups, about 1/3 cup each, filling them three-quarters full.
Bake 22-25 minutes, until the tops spring back when touched and a toothpick inserted in the center comes out clean.
Cool in the pan 5 minutes, then transfer to a rack. Store in an airtight container up to 4 days, or freeze up to 3 months.