[RPG] If a creature with Multiattack is affected by Haste, can it make two Multiattacks

No, an opportunity attack is a single melee attack.

From the Player's Basic Rules, page 74:

To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.

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.

No, even monsters only get one action per turn, and actions should not be confused with attacks. in the case of monsters that can attack multiple times, that's because Multiattack is one action, that just happens to result in attacking more than once. Each attack is not an attack action! Multiattack is also very specific about which attacks the action results in.

So an owlbear can indeed grapple or shove, but that grapple or shove is done by taking a normal Attack action, not a Multiattack action. The Attack action is general purpose and more flexible than Multiattack, but doesn't allow the owlbear to make more than one attack during the action.