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