"Man-O-Man" is an idiom that's mainly used to express surprise or excitement. However, if you're looking for synonyms for this phrase, you can say "Wow," "Gosh," "Gee Whiz," or "Oh My!" to express the same level of surprise and excitement. Other synonyms for "Man-O-Man" include "Goodness Gracious," "Holy Cow," "My Goodness," "Great Scott," or "Jeepers Creepers." These phrases are excellent to use when you want to express your surprise and amazement at something. Whatever the phrase you choose to use, make sure it's appropriate for the occasion and your audience, as using slang or an inappropriate language can offend people.