Fungal genera: Obeliospora [3 taxa], Obolarina [1 taxon], Obstipipilus [1 taxon], Obstipispora [1 taxon], Ocellaria [7 taxa], Ocellularia [212 taxa], Ochroconis [45 taxa], Ochrolechia [204 taxa], Ochropsora [28 taxa], Ocotomyces [6 taxa], Octaviania [3 taxa], Octavianina [3 taxa], Octospora [41 taxa], Octosporella [2 taxa], Odontia [22 taxa], Odontia [12 taxa], Odonticium [3 taxa], Odontiopsis [1 taxon], Odontodictyospora [2 taxa], Odontotrema [14 taxa], Odontura [2 taxa], Oedemium [4 taxa], Oedocephalum [26 taxa], Oedothea [1 taxon], Ohleria [7 taxa], Oidiodendron [81 taxa], Oidiopsis [52 taxa], Oidium [1987 taxa], Ojibwaya [4 taxa], Oligonema [7 taxa], Oligoporus [2 taxa], Oligostroma [35 taxa], Olivea [34 taxa], Oliveonia [1 taxon], Ollula [1 taxon], Olpidiopsis [10 taxa], Olpidium [34 taxa], Olpitrichum [8 taxa], Ombrophila [25 taxa], Omega [5 taxa], Ommatospora [1 taxon], Ommatosporella [1 taxon], Omnidemptus [2 taxa], Omphalia [32 taxa], Omphalia [9 taxa], Omphalina [45 taxa], Omphalopsis [15 taxa], Omphalospora [5 taxa], Omphalotus [7 taxa], Oncobasidium [2 taxa], Oncopodiella [3 taxa], Oncopodium [23 taxa], Oncospora [46 taxa], Oncosporomyces [2 taxa], Onychophora [1 taxon], Onygena [9 taxa], Oomyces [1 taxon], Oospora [26 taxa], Oothecium [14 taxa], Opegrapha [324 taxa], Opegrapha [8 taxa], Operculella [5 taxa], Ophiobolus [139 taxa], Ophioceras [14 taxa], Ophiociliomyces [6 taxa], Ophiocordyceps [5 taxa], Ophiodothella [55 taxa], Ophiognomonia [4 taxa], Ophiomassaria [4 taxa], Ophionectria [60 taxa], Ophioparodia [1 taxon], Ophiosphaerella [17 taxa], Ophiostoma [322 taxa], Ophiostomella [3 taxa], Ophiovalsa [19 taxa], Oplophora [2 taxa], Oramasia [2 taxa], Orbicula [4 taxa], Orbilia [281 taxa], Orbiliaster [2 taxa], Ordus [2 taxa], Oreophylla [1 taxon], Oropogon [13 taxa], Orphanocoela [1 taxon], Orthobellus [1 taxon], Osmoporus [80 taxa], Ossicaulis [13 taxa], Osteina [5 taxa], Ostracoderma [4 taxa], Ostreichnion [5 taxa], Ostreola [6 taxa], Ostropa [17 taxa], Ostropella [4 taxa], Oswaldoa [7 taxa], Otidea [8 taxa], Otthia [123 taxa], Otthiella [1 taxon], Oudemansiella [65 taxa], Ovularia [186 taxa], Ovulariopsis [91 taxa], Ovulinia [1 taxon], Oxydothis [12 taxa], Oxyporus [128 taxa], Ozonium [43 taxa]