Nitrous bonnet 
Niveoporofomes spraguei 
Northern Cinnabar Polypore 
Northern Tooth 
Oak Bracket 
Oak Curtain Crust 
Oak Leaf Blister 
Oak mazegill 
Oak-loving Gymnopus 
Oak-stump bonnet cap 
Oakbug Milkcap 
Oakman Ringless Amanita 
Oakmoss 
Ochre Cushion 
Ochre Gillgobbler 
Ochre Spreading Tooth 
Ochre-gilled barefoot lepidella 
Ochrolechia yasudae 
Octopus Stinkhorn 
Old man of the woods 
Old Potato Amanita 
Olive Salver 
olive-capped suillus 
Omphalina pyxidata