This has the same mistake as the Welsh Duolingo. It's not o 'clock. It should be o' clock, as the o' is assumed to be short for of. However the accepted spelling seems to be o'clock. Either way, o 'clock is wrong.
It is o'clock that we have, as that is the standard English spelling. If you're seeing o 'clock then that's because of a technical bug. :)
Tapadh leat. It is the first time I have seen this explained even though people complain both here and on the Welsh course.
Are you seeing o 'clock on the tap tiles exercise? We do have a wee issue with punctuation on those ones.