The “d” gets contracted to the word “œuvre” due to the French vowel rule, but you might already know that!
Why the contraction? Well here’s a little explanation:
Consonant words such as de, du, le, la, que, je, me, te, ne, se, or ce get contracted with an apostrophe and lose their end vowels when the following word starts with a vowel (a, e, i, o, u, y), a silent H or Y pronoun.
Note: Qui (who) never gets contracted.
Le + ocean = l’océan (the ocean)
La + amour = l’amour (the love)
De + or = d’or (of gold)
Le + homme = l’homme (the man) — Homme with a capital H signifies “men,” as in humankind.
And here’s our example:
Les hors d’œuvre étaient vraiment délicieux. (The hors d’oeuvres were very delicious.)
Note: In French, the word hors d’œuvre is invariable, meaning that it doesn’t take an “s” in the plural!