Operculum of nostrils, [11];

in Caprimulgidae, [415];

in Columbae, [325];

in Plovers, [270];

in Procellariidae, [60];

in Rhinochetus, [264]

Opisthocoelous, [6]

Opisthocomi, [186], [241]

Opisthocomidae, [186], [241]

Opisthocomus, [186], [241];