This is what I have been told, but ask someone who might know better. On'd oes is a contraction of Onid Oes (not much of a contraction really). Onid means if..not(unless, except) and comes from os nid - if not. add in the Oes and you have the "if there is not" - or as it is very frequently used at the end of a sentence it simply expresses - isn't there, aren't there. I have always thought that it's used bit more widely and loosely than that to include aren't you etc, but don't take my word for that. I'm wondering myself now.
These ones are a bit slipperry I think and you need someone here who's a confident, natural native speaker (I'm not) to point you in the right direction, although I bet many people haven't actually thought about it very much and tend to say these things when they feel right. There are a host of similar things like this, which really are the icing on the cake sorts of bits and pieces, that make sentences sound very natural, when people say them. I don't know how half of these things are actually spelt or where they come from, because you hear them more than you see them in print.
To me these things sound like: On'd yn nhw, on'd yw e (ynde fe, ond yfe - sometimes the "d" is hardened to a "t" and lots of variability by the sounds of it). Also "Ife" at the start of sentences - (ife dy blaa..blaa. blaa di yw rheina?), for no particular reason that I can fathom, apart from style, but I need advice on all these sorts of things.
These things have been embedded in the language for a long time and not recent innovations like perhaps - t'mod; ch'mod, 'sti, 'sdim etc, which I suspect might have come in the last hundred years or so, but who knows when they came in?.
I found this extract from the William Morgan Bible of 1588: (Job 7.1)
1 Onid oes amser terfynedig i ddyn ar y ddaear? onid yw ei ddyddiau ef megis dyddiau gwas cyflog?