Am I right in assuming that όι is pronounced like the oy in boy and that οί is pronounced like the ee in feet? In other words, this is the difference in placing the accent on the first letter or the second letter of a diphthong.
Exactly. When the accent is on the first vowel, it's the diphthong /oi/ as in "boy"; when on the second, it's the single vowel sound /i/ as in "feet".
Similarly for the digraphs αι and ει -- when accented on the first vowel, they are diphthongs: /ai/ as in "pie" and /εi/ as in "say"; when accented on the second, they are the single vowel sound /ε/ as in "bed" and /i/ as in "feet".
By the way, just to be technically accurate, these are diphthongs only when the accent is on the first vowel letter; when on the second, they are digraphs (two letters which combined represent one sound).