If I was to write: I will be sitting all day on open space would that be correct or should I say I will be sitting all day at open space or otherwise. By openspace I mean the typical corporate space where employees sit and work
Because it's not a proper noun, your "open space" likely needs a definite or indefinite article. If the audience of your message understands that there is one specific open space to which you are referring, use the definite article ("the open space"); otherwise, use an indefinite article ("an open space").
As to which preposition to use, you would generally say that you sit in a space.
I will be sitting all day in the open space.
You would say that you sit on a piece of furniture, like a chair or a bench, or you would sit on the floor, the ground, or the lawn, as the case may be.
You would sit at your desk, or at a landmark, perhaps, or any other place that you are physically next to but not precisely "in" or "on."