Here's a breakdown of how you could represent it phonetically:
Classical Greek:
* Ancient Greek: /oɛˈnomaʊs/ (with a short "o" sound and a long "a" sound)
* Modern Greek: /oˈɛnomaʊs/ (with a long "o" sound and a long "a" sound)
English:
* Traditional: /iːˈnɔɪ.məs/ (with a long "e" sound and a diphthong "oi" sound)
* Modern: /ˈoʊ.nə.mæs/ (with a long "o" sound and a stressed "o" sound)
Ultimately, the best way to pronounce "Oenomaus" is to consult a reliable source on Greek pronunciation or to listen to a recording of someone speaking the name.