Sorry, I'm not sure. It probably isn't important to understand them. If you go on reading, some of them are explained a few paragraphs later. "SNO" means "senior naval officer." "HYPO" might mean simply "below." UW and BD probably don't mean anything. I think they are meaningless codes, to prove identity. The arriving reinforcements will "challenge" by sending the code "BD" and they are supposed to prove they are the right ships by answering with the "response" "BD."
An "escort group" is some kind of naval term for an organized team of several warships that are supposed to sail with a convoy, "escort" them, and protect them from enemy action.
I agree with your guesses about OP ORD and 378-42.
I read this book a long time ago. I really enjoy C. S. Forester. He's most famous for a series of books about a fictitious naval captain, "Horatio Hornblower."