With the recent addition of the revisited artificer class to 5e, I did a quick playthrough with a level 3 artillerist. The artillerist can summon a turret; an object (not a construct) with a speed of 15 and they can command the turret to move as part of a bonus action. When I attempted to move the turret out of melee range of an enemy NPC, the DM tried to make an attack of opportunity against it. I pointed out to him that opportunity attacks are made against creatures, and the turret is not a creature.
This got me thinking about whether or not you can make opportunity attacks against other objects that move, such as a vehicle or even a rolling stone.
Best Answer
Objects do not trigger Opportunity Attacks
Your assessment is correct. The rules for OA state that (emphasis mine):
As you can see the rules are very clear. Unless something is a creature it can't provoke an OA.
Further, the turret created by the artillerist is not a creature but an object. From the UA:
Thus, the turret also can't provoke an Opportunity Attack.
It may seem reasonable for a DM to want to change the Artificer turret to a creature (likely a construct is appropriate). However, do keep in mind that this changes a lot more than just Opportunity Attacks. There are a number of spells and abilities that can only affect either creatures or objects or work differently with them.
With the release of Eberron: Rising from the Last War nothing changes
This question was asked about a playtest UA version of the Artificer which has now been officially released in Eberron: Rising from the Last War. The wording of the artillerist turret has changed, it is called an eldritch cannon now. However, it is still an object:
Therefore, this version of the artillerist cannon also does not provoke Opportunity Attacks, as explained previously in this answer.