Visa -- the litter came down with parvo and the breeder spent $2000 trying to keep them alive. Visa literally owes her life to a Visa card. The other survivors were named Cash and Amex.
Oliver -- When he was a baby he was tiny and submissive. His brother easily overpowered him for food the first time I fed them. He looked at his brother as if to say "please sir, can I have some more?" So I called him Oliver Twist.
Tobias -- no reason really, I just like the name, it's different and I think it's a good name for a rat.
Caleb -- I just like the name. I've always liked people names for animals (especially rats though).