1. An expression said when one is excited, pleased, or pleasantly surprised. Oh boy, a lot of people sure came to the fundraiser! A: "We're going to spend summer at Grandma and Grandpa's lake house!" B: "Oh boy! That will be swell!"
2. An expression said when one is frustrated by, exasperated about, resigned to something bad or unpleasant. Oh boy, another surprise inspection. I sure do love these. A: "Hi, Mr. Peterson? I need you to come in to the school and have a talk with us about your son." B: "Oh boy, what has he done now?"
exclam. Wow! Oh, boy! What a mess!