Here's my quick take on the issue: the jews made everything way too complicated with all their little rules about everything. Jesus came and explained that its not really that difficult; just love god and each other.
I wouldn't say that made the OT "null and void" tho. Its more like it made it unnecessary.
IMO, it became more complicated when the leading Jews figured perfect adherence to the laws would get them their nation and temple back.
How'd they figure that? What, if we really really obey the laws then God will give us our nation and temple back?
They developed the fence, which was extreme in some cases.
But there were religious reasons too, rather than just political ones, no?
Before that I don't feel they were any more complicated than any other nation's laws.
But Jesus wasn't talking about not needing national laws. He was talking about the religious aspect. It doesn't really matter if what you're doing might technically count as work and you happen to be doing it on Saturday, just love God and each other.