"I want oranges."
Translation:Dw i eisiau orennau.
The GPC dictionary uses orenau: http://www.geiriadur.ac.uk/gpc/gpc.html?oren
Gweiadur uses orenau: http://www.gweiadur.com/en/Pawb/oren (free registration required)
So I'm not sure whom to believe, though on balance, I'd trust GPC most.
I wouldn't trust GPC as most authoritative as it includes some older spellings as well as more modern ones. For modern spelling conventions The Bangor Dictionary, The National Terminology Portal and especially Byd Term Cymru would be more up-to-date. Can't find the plural of oren in any of them, mind!
I've seen both spellings but orennau* seems more popular than orenau** in practice.
isio is basically just another spelling of eisiau that you may come across in the north. eisiau has many pronunciations depending on the region, e.g. ishe, isho, and isio is one of those regional pronunciations that is also frequently seen in writing.
And dwi is dw i written together, which is also sometimes found in writing.
Some people even write the whole dwisho/dwisio as one word to match how they say it, but there's a limit to how colloquial an expression Duolingo accepts :)