If you get drafted onto a team, each week your team will be matched up against an opposing team. If your manager decides that you'll be playing ORAS in a certain week, you will be playing the opposing team's ORAS player in the corresponding battle slot. Here's an example lineup:
Bloom Doom Glooms lineup (vs Petalburg Porygons):
SM1: Player A
SM2: Player B
SM bo3: Player C
ORAS1: GlimpseOfAPast
ORAS2: Player D
BW1: Player E
BW2: Player F
Monothreat: Player G
In that example, you would be battling whoever the Petalburg Porygons picked for their ORAS1 slot. All team managers need to submit their lineup to the host(s) before the week can begin, so neither team knows for sure what their opponent's lineup will look like until all the lineups are submitted.