The Super Robot Wars series is a crossover strategy RPG featuring many robots from many Japanese works. Big O is prominently featured in Super Robot Wars D and the Super Robot Wars Z series. In fact, its plot gets played out in Super Robot Wars D, the first Super Robot Wars Z game and Super Robot Wars Z3: Jigoku-hen.
There are also some interesting crossover moments. Roger Smith becomes best drinking buddies with Rand Travis (one of the protagonists in Super Robot Wars Z) and Gadlight Meonsam (primary antagonist of Super Robot Wars Z3: Jigoku-hen). Beck is a recurring foe in the Z series, showing up in 5 of the games. The reason why Beck doesn't show up in Rengoku-hen is that Rengoku-hen is a side story focusing on the Original Generation characters and what they were up to during Jigoku-hen. Beck is playable for some of the games, and he is even a secret in Tengoku-hen. Schwarzwald also plays a prominent role in Super Robot Wars D (the one where he sides with the Ruina), Super Robot Wars Z, and Super Robot Wars Z3: Jigoku-hen (the one where he somehow came back to life).