I'm trying to use http://www.uesp.net/wiki/Oblivion:Reflect_Spell +100 against an OOO boss - but I notice that at least his Dispel and Silence spells work just fine even so. A bit surprised as at least Silence should be considered an "hostile spell" in my opinion. Does anyone have details on what's reflected and what not with Reflect Spell?

All spells cast by NPCs and creatures should be reflected.

Nonetheless, some of them are definitely not. My char is hit with (at least) Dispel and Silence w/o them being reflected. Don't think I'm using any mod that would affect this either.

Nothing consistant. Each spell has a checkbox in the CS to say whether it can be reflected or not, Most standard spells can be reflected, but it is quite possible that an OOO boss would be set up to have spells that can't.

Thanks, yes, that was exactly it. Several of his spells have the "Disallow Spell Absorb/Reflect" flags checked. Ouch!