These clearly run large- a 26 inch waist is typically a size 4 in Old Navy jeans. And these were my daughter's jeans. She wears a size 2 or 4. 

Features: High Rise Light Wash Classic 5 Pockets style with Button Fly Closure Straight Leg with a Cutoff Raw Hem Distressed Details at the Pocket Edges Material: 76% Cotton 24% Lyocell 1% Spandex