- Dendrobium obcordatum
- Dendrobium obliquum
- Dendrobium oblongimentum
- Dendrobium oblongum
- Dendrobium obovatum
- Dendrobium obrienianum
- Dendrobium obscureauriculatum
- Dendrobium obtusum
- Dendrobium ochraceum
- Dendrobium ochranthum
- Dendrobium ochreatum – Stem-clasping-tube Dendrobium
- Dendrobium ochroleucum
- Dendrobium odoardii
- Dendrobium odontopus
- Dendrobium odoratum
- Dendrobium okabeanum
- Dendrobium okinawense
- Dendrobium oligadenium
- Dendrobium oliganthum
- Dendrobium oligophyllum – Few-leaved Dendrobium
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- Dendrobium olivaceum
- Dendrobium oppositifolium
- Dendrobium orbiculare
- Dendrobium orbilobulatum
- Dendrobium oreodoxa
- Dendrobium orientale
- Dendrobium ornithoflorum
- Dendrobium osmophytopsis
- Dendrobium ostrinoglossum – May River Orchid, Sepik Blue Orchid
- Dendrobium ostrinum
- Dendrobium ostrinum var. ochroleucum
- Dendrobium ostrinum var. ostrinum
- Dendrobium otaguroanum
- Dendrobium ovatifolium
- Dendrobium ovatipetalum
- Dendrobium ovatum
- Dendrobium ovipostoriferum
- Dendrobium oxychilum
- Dendrobium oxyphyllum
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