This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
勜 combines 翁 (old man) and 力 (strength). The exact historical development is uncertain, but it likely relates to the idea of an old man's strength or vigor.
An old man (翁) with strength (力) suggests vigor, but the exact meaning is unclear. Picture an elderly person still exerting force.
For オ, imagine an old man (翁) letting out a strong 'Oh!' (オ) as he uses his remaining strength.