I haven't run into any words that have oi in them that I recall. Is it a valid construction? How is it pronounced if so?
Searching Tekstaro for “oi”, I found a lot of examples of words with that vowel combination. Here are some examples:
As for pronounciation, as usual in Esperanto, it is “read what you see”. :) That is, it should be clear that it is “o” then “i”. One should not read it as if it was “oj” but I cannot give much more advice as I am not much of an expert on explaining pronounciation.
There is a syllable boundary between the "o" and the "i" ("i" is the next syllable in fact). This is especially noticeable when the "i" becomes stressed, and even more noticeable when the speaker lengthens stressed syllables. A diphthong would be written "oj" and considered a vowel+consonant phonemically.