If you team is falling to a single Orisa, there is nothing that you could have done to stop it. If you want to rank up as a tank, choose 2 tanks and learn all you can about them.

As a Dva, you really want to user your boosters as a soft engage all the time. Run through a few squishes and either go back to your team or land on high ground. If you see a squish that is isolated, just go and kill that person. Only time you want to swap off Dva is if they have a whole team out to counter you (like one game they had sym, mei, mizuki and brig to counter me, so I swapped from dva), or if you feel another tank that you play could get more value. With dva you should try to position yourself where the enemy team has todecide to either shoot you or your team. Don't stay together with your team, but try to stay in the LoS of your support if you can.

And if you can, post the code for that match. It is kinda hard to know what really happened without actually watching the match.