Usually system mechanics have to be made OGL, this is because they are derivative work from the SRD and can only be published undedr the OGL, otherwise they would have to be made your house rule and posted in websites like it was until the SRD came, the copyright stuff would prevent any publishing.
It does not matter if the work is or is not original, anything using the SRD mechanics, thus monsters stats, new abilities given to them, their names, types, sizes, feats, skills, are necessarily OGL, even if you do not say so in the OGL, they are to be and you will not have a case.
That said, description can usually be made closed content, but you would want to have some bits of it open, this is what WotC has done to the monsters in the MM when they went to the SRD, check that out for an idea. I advise you to use italics for anythng not closed content in the monsters, making sure not all of it is either closed or open.
I know Monte has maded some copyright claims over a specific way of writing something, that is legally sound but not stone solid though, and so it can be broken in a court, if it ever goes that way, I doubt because Monte is very polite and usually address problems without the need to pay a lawyer.
That all said, I cannot really say whether the whole work or not must eb OGC, but I know that it is hard to avoid any mechanics being, pantheons are hardly ever open, but their stats, if given, those are open. I could go on for sometime, but I have the OGL and nothing of the bestiary to specifically point things out.
Anyway, I am not a lawyer, yet, and I am not frrom either the UK or USA, but copyright stuff is one of the most world wide laws, since you do want it to work well all around it.
