Are these expressions used universally throughout Wales?
Which expressions? On'd yw e? is used as a tag (Mae e'n dal, on'd yw e? "He's tall, isn't he?") in parts of the south.
Would on'd ydy o? be used in the north?
The North Wales variant, from what I remember is: On' tydy?
Where the 'd' has hardened a little.
I thought it was just 'tydy' in the North.
Yes, that's a very common contraction.
We (Southerners) laugh (in a friendly way!) at our friend from Blaenau Ffestiniog because she's forever saying "ynditydi" "it is, isn't it?". She says it really quickly and when we pointed it out to her she didn't even realise she said it!
The 'O' can be replaced by a 'Y' in North Wales e.e. yndydy, yndwyt