I just figure opp power works the same way opp damage
ex: phonos 7/2 ->opp dmg if he fight a 1/1 card he will do 2 dmg (not 1) . That i am sure.
so Python X Selsya both will have 3 power (selsya will get Python power but Python will stay with his power since its bigger then the opp)
again, not sure about opp power but i think it works same way then opp damage