I’m gonna have to agree with Pandemic Punch on this one. Once you forced him away you should’ve turned your attention on the minions and not chased him. You have to remember that Darius has %armor pen built into his kit as well, he like killing tanks.
You did well to force him off, but that should've been the end of that fight, especially since you used ult for no confirmed kill. If I were you in that spot, I would’ve saved that first Q for after rooting him to knock him closer to the turret, then ult if you wanted to cut off his escape and take more shots or try to force a flash. In a Tank vs Darius, the tower is usually your best friend and you rarely want to chase them out into the middle of the lane unless allies are coming to help.
Edit: don’t get me wrong, I understand tanks are rough (Ornn is one of my main champs after all), but that video mainly shows a bad chase call.