The bunnies need the oat hay. Oat hay is abrasive, it contains silicates, so it helps keep the rabbits teeth from getting too long...remember rabbits teeth are constantly growing and they need things to chew to grind down their teeth.
Any grass hay will wear down a bunny's teeth. It can be oat grass hay, Bermuda grass hay, orchard grass hay, timothy hay, etc. It is the chewing of hay (or grass) that causes a bunny to move its jaw in a figure 8 motion. It is this motion that grinds down the teeth.
Chewing other things like pellets, wood chews, or sticks does not grind the teeth the way hay chewing does.