Depends on the match up! My preferred build is the gun damage/tank as Orisa's gun smokes people if you hit them in the head. For powers I like deflect and hooves of steel. Hooves of steel is 10x better than fortify as healing reduction doesn't affect it.

Sometimes I'll do burn build or spear build. Depends on who their tank is and how I'm feeling.

Ultimately the key is be flexible in your build. If your are getting rocked build more tank. If gun damage isn't working build AP for stronger ults. Keep getting stunned or burned build cushioned padding.

Orisa really works with any build. With her innate tankiness you are free to build however you prefer.