No, an opportunity attack is a single melee attack.
From the Player's Basic Rules, page 74:
make the opportunity attack, you use your reaction
to make one melee attack against the provoking
Additionally, multiattack specifies that it can't be used in this way. From the DM Basic Rules, page 6:
A creature that can make multiple attacks on its
turn has the Multiattack ability. A creature can’t use
Multiattack when making an opportunity attack, which
must be a single melee attack.
Multiattack can be used only on the creature's turn, and it explicitly can't be used with opportunity attacks.
Finally, if you look at the statblock of a creature that has Multiattack, you can see that it is listed under the Actions section of the statblock. Multiattack is a specific action that a creature can take, just like a player can take the Attack action or the Use An Object action. Just like a Fighter's Extra Attack, you can't use it every time you make an attack. A Fighter can only use Extra Attack when they take the Attack action, and a creature can only use Multiattack when it takes the Multiattack action.