Renas,
What Open Graph plugin are you talking about? The category drop-down for Content, K2, and EasyBlog will work for any sub-categories as well. So, if you choose the parent category for a large number of sub-categories, all of those sub-categories will be included in that Object Type. If you create a Parent Object and then a new object type for one sub-category only, it will work as well so that all categories get type1 except for the one sub-category with type2.
There's a lot more to do in our documentation and we plan to create some videos demonstrating this, but I hope it helps out. In general, yes, you may need to create a few Object Types for a few parent categories, but you shouldn't need a new object type for each category on your site.
Let me know if that helps, or if you need more information.
Thanks,
Alex