Place chicken in a 4- or 6-qt. electric or stove-top pressure cooker. In a small bowl stir together salsa, oregano, and garlic powder. Pour over chicken. Lock the lid. Set an electric cooker to cook 15 minutes. For a stove-top cooker, bring up to pressure over medium-high heat; reduce heat enough to maintain steady (but not excessive) pressure. Cook 15 minutes. Remove from heat. For both models, release pressure quickly. Open lid carefully.
Transfer chicken to a cutting board, reserving liquid in cooker. Use 2 forks to shred chicken into large pieces. Transfer to a bowl. Spoon enough cooking liquid over chicken to moisten. Divide chicken among roll bottoms. If desired, sprinkle with queso fresco. Top with chopped cilantro and shredded lettuce. Add roll tops.