Preheat oven to 350°F and line a baking sheet with parchment paper. If using whole oats, pulse in food processor until flour consistency.

Crack egg into medium mixing bowl and whisk until smooth. Add Greek yogurt and combine thoroughly. If using peanut butter or cinnamon, incorporate now.

Gradually add oat flour to wet mixture, starting with 1.5 cups and adding remaining slowly until workable dough forms that holds together without being too sticky or dry.

Transfer dough to lightly floured surface and roll out to 1/4-inch thickness. Use cookie cutters or cut into squares. Re-roll scraps to maximize batch.

Arrange shaped biscuits on prepared baking sheet, leaving about an inch between pieces. Bake 15-18 minutes until edges are golden brown and biscuits feel firm.

Remove from oven and let cool completely on baking sheet. Store in airtight container once completely cooled.