While I've never used oak bark because it has never been available to me, it's pretty resistant to rotting, and if you don't over-sanitize it, should come up with some mosses and stuff on its own. I think you could probably peg some epiphytes to it such as the Neoregelias. It has a lot of tannins, which are actually "good" for the frogs.
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Patty
Pahsimeroi, Idaho

Dendrobates: auratus blue, auratus Ancon Hill, tinctorius azureus, leucomelas. Phyllobates: vittatus, terribilis, lugubris. Epipedobates: anthonyi tricolor pasaje. Ranitomeya fantastica, imitator, reticulata. Adelphobates castaneoticus, galactonotus. Oophagia pumilio Bastimentos. (updated systematic nomenclature)