R.Ost is among the most well-equipped pushers in the game. He has fast movement speed, high health, and a rapid area attack that will shred your cats. As the icing on the cake he can do Critical Hits, which usually is the difference between whether or not he oneshots Eraser Cat, and does not have a trait making it hard to inflict status effects. He has five(5) knockbacks, so be sure to bring lots and lots of high damage dealing units, and lots of meatshields, as that's pretty much all you can do to stop him.
Strategies
First form Bahamut Cat might be able to help in earlier stages if you do not have ranged decently-leveled attackers such as Dragon Cat or Crazed Dragon Cat.
Awakened Bahamut Cat can be used to chip off much of a 100% buffed R. Ost, then let it be finished with the rest of your attackers.
Dark Mitama can easily perma-slow this avian atrocity from long distance. The weaken effect helps a lot too, so don't be afraid to save an extra slot for Thaumaturge Cat.
Dragon cat, Macho legs cat, Manics macho legs cat and manic king dragon will do fine against R.ost
Ostrich club. By the way in until the ostrich is the same last name, good meeting is the name. Sometimes it unleashes critical.
Variants
Golden R. Ost (Event Enemy)
A Metal enemy that only appears in the Golden Week and Silver Week events. It drops less money and has the usual Metal ability of only taking 1 damage except for Critical Hits, but it has much lower stats all across the board and cannot perform Critical Hits itself.
M. Ost (Relic)
The Relic version of R. Ost, with even higher health, damage, range and speed, but many knockbacks. It has a 100% chance to Curse and knock cats back, and slows cats with each hit.