Can the spiritual weapon from the spell conduct an opportunity attack?

Question

This question arose last night when our cleric had moved the spiritual weapon in pursuit of an enemy well beyond the battle's front line. The enemy is at the far right of this illustration, and is about to move off the screen to the right. (The spiritual weapon is represented by a cat token ... flavor for a nature domain cleric).

The question that arose was: can the cleric make an opportunity attack with the spiritual weapon as the wight moves out of its reach? The wight was within a 5' reach of the weapon, but was not within reach of the cleric who is back by the main melee.

The ruling was no, since the weapon is not a creature and thus its reach is moot. That seemed a reasonable ruling, but the question that arose as we discussed it afterwards was:

Did we miss out on a rule, or ruling, that would have allowed that spiritual weapon to make an opportunity attack on that wight?

Answer 1

No, Spiritual Weapon can't make Opportunity Attacks.

Spiritual Weapon is a spell cast as a Bonus Action, that then allows you to use a Bonus Action on subsequent turns to make additional attacks, as below:

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.

Since Opportunity Attacks rely on using a Reaction, there's no provision given by the rules for this to happen with this spell.

That said, as with almost everything else in D&D, your DM may rule that it's ok, and that'd probably be fine in this case.

Jeremy Crawford also agrees and says that spiritual weapon is not intended to be able to make opportunity attacks in this Tweet:

The spell's description would say if the spiritual weapon made opportunity attacks. It intentionally doesn't.

Answer 2

No, especially not according to RAI

The rules as intended (RAI) are spelled out by Jeremy Crawford in a twitter exchange

Ed Robinson: Q that came up at game tonight, does spiritual weapon threaten the square it's in for attacks of opportunity?

Jeremy Crawford: The spell's description would say if the spiritual weapon made opportunity attacks. It intentionally doesn't.

Note that this wouldn't even work with the Warcaster Feat (which allows spells to be cast in the place of Opportunity Attacks) because that feat requires:

The spell [used to target a creature in place of an opportunity attack] must have a casting time of 1 action and must target only that creature. (PHB p. 170)

Leaving aside for a moment whether or not Spiritual Weapon targets only one creature (it doesn't, but even if it did), it definitely has a casting time of one Bonus Action, so it still wouldn't apply.

Answer 3

No, spiritual weapon cannot be used to make opportunity attacks

Rules as Written

Spell do only what they say they do in 5e. Note that this spell does not summon an ordinary weapon that you can use in whatever way you would use one. Instead, it creates a spectral weapon whose uses are clearly defined by the spell text.

The spell describes the only way to attack with the weapon after the initial casting as:

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.

There is no mention of any way to use the weapon to attack using a reaction for an opportunity attack, thus it simply does not have the ability to be used in this way.¹

Rules as Intended

Jeremy Crawford agrees and says that spiritual weapon being unable to make opportunity attacks is completely intended in this Tweet:

Q: [D]oes spiritual weapon threaten the square it's in for attacks of opportunity?

Crawford: The spell's description would say if the spiritual weapon made opportunity attacks. It intentionally doesn't.

Allowing this would make the spell more powerful

Action economy

As written, the weapon can only make an attack as a bonus action on your turn. By opening up the option to use the spell as a reaction you have increased its utility. Firstly, by allowing this it can now be used to attack outside of the caster's turn.

Secondly, it would then be allowed to be used as a reaction instead of a bonus action. Reactions and bonus actions are different in their uses and restrictions. Opening up the option here might allow a character to save their bonus actions for other class abilities by using this spell as a reaction for example.

Thus, by giving the spell more ways to use and and more utility you increase the power of the spell.

Area control

Allowing this weapon to be counted as an extension of the caster's reach provides a doubling of the area that the spellcaster threatens and doubles their opportunities to provoke OAs.

Additionally, the weapon would be very effective at threatening OAs since it cannot be attacked. Thus it can move freely into melee territory and lock down enemies with no risk to the weapon itself. This is much more powerful than another creature that can be hit positioning itself within enemy melee range. The caster doesn't even have to give up the ability to make OAs from their space to do this.

From a pragmatic standpoint this also would mean that two separate reaches would have to be tracked throughout the battle for that one character which could get unwieldy and potentially could bog down battle a bit.

Up to the DM

As always, even with these concerns the DM can allow it. Just note that the spell will be more powerful and that it is going to go against the intent and RAW for the spell.

---

^{1- Having the Warcaster feat would also not change this as the feat only allows for casting a new spell in place of an opportunity attack (not using a currently active spell to make an OA) and also it only works for spells with a casting time of 1 action (spiritual weapon is a bonus action spell).}