Since "Oreille" is feminine why "mon" in my response version (rather than "de l'")?
With body parts, possessives are not used if the owner is obvious. In this case, since you are not supposed to hear with someone else's ears, you don't need "mon".
Got it! Thank you.
Why "de l'" rather than "a l'"?
Preposition may not have rhyme or reason.
You may consider that "de" means "from" in this sentence.
mon oreille means it's masculine does it not? If so why is droite rather than droit correct?
"Mon" is used with masculine singular nouns, yes, but also with feminine singular words starting with a vowel sound.
"Mon oreille droite" is feminine.