The OGL specifies that "all derivative material" published under the OGL
must be Open Content. So Paizo does not have a choice for a lot of their content, including the Core Rulebook. Period.
(They don't have to host their own hyperlinked SRD though...)
However, I've never seen a very good definition of what "derivative content" means, or where the line is drawn.
At the very least, anything based on established material, such as the statblock of a troll fighter, is derivative content. It's very likely that anything that expands on established material, such as a fighter archetype or a feat that uses an Power Attack as a prerequisite, is also be derivative. It could also be argued that making new content that is based on established rules, such as completely new feats or spells, is also derivative, as it is very obviously based on established mechanics.
But after that things get a little blurrier. Is something like a new class derivative? Or a monster? Or a new mechanic only loosely based on existing rules, like the Words of Power system?
@
Mistwell might be able to clarify this as it's likely one of those things were the legal definition is not the same as the plain language definition.