Crouching next to Orisa will allow you to ride Orisa. There are 3 heroes available to ride Orisa in this mode and they only have melee weapons enabled. Eliminate all of the opposing Orisas and their riders to win. When not on an Orisa riders have low health and move very slowly.
The heroes available are:
Orisa - A glorious steed to ride into battle on, can use every ability except primary fire and shield, does very low damage when not paired with a rider
Reinhardt - Medium range and damage, a good general damage dealer
Torbjorn - Very short range and very high damage, can use overload for even more damage, but has to be very close
Brigitte - Longest range with low damage, has healing abilities