Roscar is the ended relationship and formerly engaged couple between Remi and Oscar.

We'll find each other again... on the other side of all of this. I promise.
Remi to Oscar
Remi and Oscar met within Sandstorm where Oscar served as an operative. According to Jane's memories, Remi developed strong feelings towards Oscar; she seemed to rely on him, finding in Oscar a partner who lately became her fiancé.

Right after Remi and Shepherd came up with Sandstorm's ultimate plan and before she submitted herself to the tattoo and memory-wipe process, Remi broke her engagement to Oscar but promised that they would find each other again. It's implied that they decided for Oscar to become Remi's handler once they met after she was placed in the FBI in order to give Jane tasks to accomplish Sandstorm's goals. 

Remi and Oscar's relationship ended the moment her memory was completely erased with ZIP.