Ό,τι = Whatever, anything, what
Θα κάνω ό,τι θέλω - I'll do what/whatever I like.
Θα κάνω ό,τι μπορώ για να σας βοηθήσω. - I'll do anything I can to help you.
Ότι = that
Μου είπε ότι θα πήγαινε στο πάρτι - He told me (that) he could go to the party.
Ξέρω ότι του είπες την αλήθεια. - I know (that) you told him the truth.
I hope this is a bit helpful too. ^.^
(And I agree. I wouldn't take Oxford's word as gospel. :P)
The courses on Duolinog are created and moderated by native speakers of both the source and the target language with the proper credentials.
Oxford is a fine dictionary however, we can't expect the pocket version to be quite complete. There might be some confusion between ότι which is a conjunction and means "that"; and ό,τι a pronoun which means "whatever".
You might like this online compilation of dictionaries we find reliable: https://www.lexilogos.com/english/greek_dictionary.htm#
Don't forget to check out the links in the Greek Forum for other sites.
0 in Greek is μηδέν.
(The English word "zero" comes from Italian and ultimately goes back to Arabic via Latin. No Greek involved.)
ζ and ξ are separate letters in Greek, if that is what you are asking. The first sounds like /z/ as in "buzz", the second like /ks/ as in "bucks".