In our game last Friday something came up that was strange.  The group was not doing well in the fight but trying to push throw anyways.  The fighter killed the person he was fighting and was rushing to get to the next person.  The Cler (one of the other players) that was standing near the path and though of something crazy but smart.  He turned to be and said "Since me and him fight all the time about religion that means me and him are hostal to reach other."  Not sure where he was going with this I told him to go on.  He then said " since he is hostal and moved throw this area I get a attack of opportunity on him.  Since I have War caster that means I can cast a spell so I cast Cure Wounds."  I told him that is not how that works but I will allowed it as a one time trick.  

What do you all think about it would you have allowed it.  I am even thing of allowing it going forward and I want to know what you all think about it.  

No, that is an absurd proposition. Hostile refers to "intending to do harm", in the context of the current conflict.

Comparing social antagonism to war is like comparing siblings yelling "they're breathing my air" to committing fratricide.

An "ally" (intending to support) can not simultaneously be classified as "hostile", with the exception of some grey area around magical charm effects.

That is definitely bending the rules to power game, and I wouldn't allow it.

No.

You have exactly two internally consistent options.

1)  Keep the RAW:  everyone miraculously knows whether or not someone else is hostile by letting them walk away.  Like every other ability in the game, they're entitled to know what they need to know for their abilities to resolve.  No amount of deception, disguise, polymorph, etc can hide this. 

2)  Drop the word "hostile" from the RAW and replace it with text of your choosing, which could be the empty string (i.e. just deleting it), house ruling for greater immersion.

So pick one.  Either everyone has perfectly accurate hostility detectors, or they don't.

Sure, fire away!

BUT whatever you do has to do damage to your target, none of that cure wounds garbage. Opportunity ATTACK has the word ATTACK in it. Cure Wounds is not an attack.

The PHB says this about hostile creature:

...hostile [creatures] are inclined to get in your way.

Not much. Luckily the DMG has a bit more:

A hostile creature opposes the adventurers and their goals but doesn’t necessarily attack them on sight.

"Lucky" may have been an exaggeration. The keen eyed (and keen minded) will notice that neither of these are a definition of what it is, only what it does. Since the rules don't tell us what it is, it must be a common language thing. So lets find a definition that fits:

Of or related to an enemy.

Openly opposed or resisting.

Ok, so basically a hostile creature obviously hates you and want you to fail.

And to answer the question of if these 2 PCs can make opportunity attacks against each other they only need to answer 1 question: "do you want to help them?" If the answer is no, then they can attack, and if they then use that attack to help them (by let's say: healing them) then they are a metagaming liar.

This effectively turns your healer into a drive-through. Instead of the healer having to endanger themselves and get into melee to deliver a large healing spell with touch, a friendly can just coordinate to pass by your reach and activate your reaction to get tagged with a heal spell. Couple it with something like tunnel fighter and you will be dishing out healing all over the place, at least until your slots run dry. Not sure if it would be an over powered adjustment to your game or just add some interesting battlefield options when things start to get tight. A friendly might have to move out of a potential opportunity attack in the first place to get healed, or be leaving a downed opponent to engage at a different location.

As stated it could be fun house rule to exercise and report back how it flavors combat differently. Did it seem to imbalance your game or make things more engaging? I would like to know how large of an impact it is in play vs. hypothetical evaluation. The boards can throw semantics all day long and specificity of rules, but the core rule of the game is you make the rules. These are just a set of guidelines which have been evaluated to be mostly balanced...'coughs in hands suspiciously'.

I might allow an OA against an ally if you really wanted to, but only if you could convince me that you bear each other some hostility and that it was something you would generally do to a hostile creature.

So, in this case, they bear each other some hostility due to arguments about religion, fine. However, if he weren't an ally, would you heal him in this situation? No, you wouldn't, so it isn't a valid OA.

Yeah, no. That's not happening. Arguing about religion doesn't constitute being hostile. Are they not still members of the same party, with the same general goals? Do they not work with each other in combat? Is the entire intent of the proposed sequence of events not intended to help their party members? They are allies.

I agree with everyone above that you can't opportunity attack someone you aren't actively fighting.

However, I'm now thinking about the rp, "You can't die till I've converted you!"(cast cure wounds) Followed by the cleric punching the fighter in the face as soon as combat ends

Hostility is in the mind of the attacker. If someone moves out of your range, you can decide on a moment's notice their actions are hostile to you and swing a sword at them. But I would draw the line at cure wounds. That is squarely in, "Come on, man" territory.

I'd allow it. 

Everyone is playing off of the semantics that an opportunity attack is against an opponent you are actively defending against who drops their guard to move away. This sets up an opening that can be exploited as a reaction during combat with a physical attack. Special focus on the wording of "hostile' seems to be the primary focus of this thread.

I have been thinking more in lines with how changing the opportunity attack to allow other actions, granted by a feat like Warcaster, would make the bonus and reaction more usable and flexible across all classes. The idea of being able to cast a full action spell as a reaction is already made possible by the feat itself. Why not have it also cover the ability to exploit a friendly moving through your guarded area. I KNOW...'with a roll of my eyes', what it says and how the word 'hostile' is used, but would it really cause a huge disruption in flow? I am just in one game and I know a bunch on this board have been dissecting every misspelling or incorrect word usage to dive into the minutia of how the meaning of a color sentence can be interpreted to undermine the intent of the whole. And when it comes down to it, every rule in the book is a house rule to be followed and interpreted with the intent of fun and creativity in mind.

Yours is a fair point, and I have been thinking about it. I think it comes down to the idea that if you want to treat someone who is ostensibly your ally as a hostile creature, then you need to justify it to your DM. And there are good reasons why this might legitimately happen. But if you do so and then turn around and want to cast cure wounds on your frenemy, you are going to need to sell that to your DM as well and at that point, it's you sitting at the game table making a persuasion check in real life toward your DM.

It definitely feels to me like a bad faith request though.

In a three-way fight, do you also rule that you can't Help the enemy of your enemy by distracting a creature within 5 feet of you, just because they aren't really a "friendly creature" or your "ally"? 

Sometimes 5E uses overly restrictive language when its authors failed to contemplate niche applications. This might be one of them, unless you think that hatred is a necessary component of reaction time. 

Ignoring the implications of an OA on an ally for a moment:

Simply put, an Opportunity Attack has to be an ATTACK. Yes, War Caster allows you to cast a spell for an OA, but RAI, it should still be an attack spell. If it isn’t considered an attack (I.E. does not deal damage, grapple, or otherwise hinder the Target), then you can’t use it for an OA.

Done ignoring the implications of an OA on an ally:

Personally, I’d allow OA on allies the same way I’d allow regular attacks on allies. (Y tho?) But again, it must be an ATTACK in order to perform an OA.

Quote from Chicken_Champ >>

In a three-way fight, do you also rule that you can't Help the enemy of your enemy by distracting a creature within 5 feet of you, just because they aren't really a "friendly creature" or your "ally"? 

Sometimes 5E uses overly restrictive language when its authors failed to contemplate niche applications. This might be one of them, unless you think that hatred is a necessary component of reaction time. 

Yes, I do rule that way because the Help action is explicitly used with friendly creatures. If you are in a three-way fight, none of the creatures on those two other sides are friendly to you.

What you are probably thinking of is not the Help action, but the optional rule on flanking which allows you to get advantage if an enemy of your target is within 5 feet of it. Flanking does not require the other creature be friendly to you; Help does.

Nah, I don't play with Flanking, I think its bad for the flow of game and leads to players agonizing over pathing and placement rather than rule of cool narratives and battle flow. I meant Help. 

I dunno, "friendly creature", "ally", and "enemy" aren't particularly well-defined anywhere in the PHB, I usually just glaze over and read them all as "creature" to avoid dumb stuff like Sanctuary having inconsistent rules about spellcasting, etc. I'll 100% concede that RAW those words mean what they say, I just don't think from a house rule/ruling perspective, that "friendly creature"/"ally"/"enemy" etc. add anything helpful to 5E. 

This is why I like the wording on sneak attack, it doesn't matter if the other person near the target is your enemy or ally, as long as they're an enemy of the target.