O’n i’n is when you want to refer to something that you did repeatedly over a period of time - I find it helpful to think of it as ‘I used to…’ or ‘I was’ - so ‘When I was a child I used to eat snails’ - Pan on i’n plentyn on i’n bwyta malwod.
Wnes i is for something that you did and then stopped doing: ‘I bought a new coat’, Wnes i brynu côt newydd.
So they can be mixed in the same sentence: On i’n cerdded lawr i stryd pan wnes i weld i tîm rygbi Cymru; I was walking down the street when I saw the Welsh rugby team.
(Edited because English spellcheck)