I dislike the term. It is one of the vapidisms of urban planning babble.
"Open space" is not "park" or "preserve" or anything of substance. It is the absence of something i.e. human buildings (and usually a real rasion d'etre as well.) It is the product of disliking buildings, or of allowing such awful buildings to be built that "nothing" is better than more of them.
Don't misunderstand. I love parks and preserves and "forests." I usually vote my tax dollars for them and I am all for putting as much of the North American continent as possible in a "lock box" until we can do a better job of building in already-developed areas.
It's just the language I find empty.
Of course there is good reason for it when the public is too cheap to authorize its government to simply pay for land for public use. "Open space" is a deliberately-vague term when used as a zoning requirement.
"No sir. We don't have any requirement to make some of your land into park. It's just an 'open space' dedication."
To be fair, such requirements usually do NOT also require that the public be allowed on the property...right now.