The Collaborative International Dictionary of English v.0.48:

Octo- \Oc"to-\, Octa- \Oc"ta-\ [L. octo eight, Gr. 'oktw`, with
   a combining form 'okta-. Cf. Eight.]
   A combining form meaning eight; as in octodecimal,
   octolocular.
   [1913 Webster]