Boy!

boy

An expression said when one is excited, pleased, or pleasantly surprised. A lot of people sure came to the fundraiser, boy! Boy, that was a delicious birthday cake!

Boy!

and Boy, oh boy!
phr. a sentence opener expressing surprise or emphasis. (This is not a term of address and can be used with either sex, although it is quite informal.) Boy! Am I glad to see you, Molly!