Y and O ???
is (y) changes to (e)when the next word begin with the letter i .... and (o) changes to (u) when the next word begin with the letter o or ho ???
2 Answers
Here's an example from our very own Phrasebook:

Credit Ray on the white elephant reference! Cheers 
More on the subject: Two of the most common conjunctions in Spanish y (meaning "and") and o (meaning "or") can change spelling and pronunciation based on the word that follows.
Both changes help keep the conjunction from blending into the following word. Y becomes e when it precedes a word that begins with the i sound, while o becomes u when it precedes a word that begins with the o sound. Typically, then, the y becomes e when it precedes most words that begin with i- or hi-, and o becomes u preceding words starting with o- or ho-. Y does not change before words, such as hierba, that begin with the ia, ie, io or iu sound, regardless of spelling. See Changes in 'Y' and 'O'
Yes to the y changing to e. As to the other I don't know but I have never seen it or at least I have never taken notice of it. But it may be correct. Wait for more answers.