Prepare cucumber, tuna, bibim paste, onion, and mayonnaise.
Julienne the cucumber.
Finely chop the onion.
Drain the oil from the tuna and get it ready.
In a large bowl, combine cucumber, tuna, onion, mayonnaise, and bibim paste. Mix well.
Taste and adjust seasoning with salt and pepper if needed.
Transfer the finished O-Cham-Bi to a plate.