I NEVER use the term "Open Marriage" to describe my relationships. It has connotations which people will assume, and may not ask you about. Polyamory is just one type of Open Marriage, and there are many more (swinging being the other major one).
I use the term polyamory, the hope being that they ask what that is, having never heard of it before. When I start to describe what it is, they usually come up with a "so, you're swingers", at which point I gently correct them.
"Listen, or your tongue will make you deaf." - Native American Proverb