When it comes to addressing someone in an informal way, "old chap" is just one of the many words and phrases that we use. Some of the synonyms for "old chap" include "buddy," "mate," "pal," "comrade," "fella," and "dude." These words may vary depending on level of formality and context. In some cases, "old chap" can be replaced with terms like "grandpa," "papa," or "old man" to refer to an older male person in a respectful and affectionate way. Whatever the term used, the intention is to show respect or build camaraderie with the person being addressed.