Although these diphthongs (a diphthong is two vowels pronounced together in one syllable) sound the same, they are not interchangeable. The OI is used in the middle of a word and must be followed by at least one consonant. Examples are appoint, poison, and coin. The OY is used at the end of a word. Examples are employ, convoy, and joy.
Are there exceptions? In the OY group, we have loyal, royal, voyage, and oyster. The first three words come from the Old French: loial, roial, and voyage. The word oyster was originally spelled oister.
Somewhere along the way, probably many centuries ago, the I in the French words was turned into a Y. Perhaps it was the decision of a scribe who refused to have three consecutive vowels in an English word. Today there are only two commonly used English words that have the consecutive vowels OIA. They are paranoia, a word invented in the 19th century, and Sequoia, the name of a Cherokee chief who invented the Cherokee alphabet and taught his tribe how to read and write their own language. This name (which was also given to the giant redwood trees) is also one of the few English words that contain all five vowels.
How about the OI group? There are no commonly used English words that end in OI and only two that start with OI. Oil comes from the Latin oleum (which means “oil”), and ointment comes from the word anoint.
The really strange words are choir and tortoise. Choir, which is pronounced kweye-er, is a combination of the Latin word chorus and the French word choeur (which means choir, chorus, and chancel in a church).’ Tortoise came to us from the Spanish word tortuga (turtle).
Finally, there is the suffix –oid, which means resembling or like. Examples are asteroid (like a star), spheroid (like a sphere), and android (resembling a human).