if i use war caster opportunity attack can i use a spell for a action

Me and my father seem to have a disagreement regarding the feat war cast which says  "When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature"  he says if I use this to do a spell as a reaction I can't cast a spell during my turn is this true

Under 5.5e rules, you can cast only one spell with a spell slot on your turn. However, by definition, a reaction does NOT happen on your turn; it happens on someone else's turn. Therefore, you can indeed cast a spell on your turn and also use Warcaster to cast a spell (which meets the requirements stipulated by Warcaster) as a Reaction .

Quote from Stabbey_TC >>

Under 5.5e rules, you can cast only one spell with a spell slot on your turn. However, by definition, a reaction does NOT happen on your turn; it happens on someone else's turn. Therefore, you can indeed cast a spell on your turn and also use Warcaster to cast a spell (which meets the requirements stipulated by Warcaster) as a Reaction .

The second sentence is not true. It's true that reactions are almost always taken on another creature's turn, but there is no restriction that they must. Counterspelling a counterspell is one of the great examples of using your reaction on your own turn.

As for the OP, though... if you are using your reaction as an OA to then turn into a spell, you are indeed taking that on another creature's turn and will be able to cast a spell with a spell slot when it's your turn as well.

Dad is wrong.

If you use warcaster feat to cast a spell as an opportunity attack, that means someone left your reach, which likely happened because they moved away from you during their turn. You can only burn one spell slot per turn, so if they moved away from you on THEIR turn, you could burn a slot on their turn for the OA. Then on YOUR turn, you could burn a slot for something else.

Note: it is possible that on YOUR turn, you burn a slot to cast Dissonant Whispers, which causes the target to use their reaction to run away ON YOUR TURN.  This means your opportunity attack using warcaster would have to use a cantrip or some non-slotted spell

Quote from sabin76 >>

Counterspelling a counterspell is one of the great examples of using your reaction on your own turn.

This is also much less likely due to 2024 rules. If you cast fireball with a slot and en enemy counterspells it, you cant cast counterspell because you already used a slot to cast fireball and you cant burn 2 slots innthe same turn.

Quote from SunIsGettingRealLow96589 >>

Quote from sabin76 >>

Counterspelling a counterspell is one of the great examples of using your reaction on your own turn.

 

This is also much less likely due to 2024 rules. If you cast fireball with a slot and en enemy counterspells it, you cant cast counterspell because you already used a slot to cast fireball and you cant burn 2 slots innthe same turn.

I agree. It's possible to do with an initial casting of a cantrip, but at that point, I'd just let the CS go through. Wasting a CS on a cantrip is very much in the cantrip caster's favor the vast majority of the time.

Quote from Angel_of_death11 >>

he says if I use this to do a spell as a reaction I can't cast a spell during my turn is this true 

99% of the time this is not true, because you'll typically be using this to cast a spell using your reaction outside of your turn and that won't affect what you can cast during your following or preceeding turn.

The 1% exception would be if the creature is somehow provoking an opportunity attack on your turn, and even then it'd depend on which rules version (5e or 5.5e) you're using and which spells you've used or want to use.

For example, under 5e rules you cannot cast a bonus action spell in the same turn as any other spell except a cantrip of casting time of 1 action. This means if you have already cast a bonus action spell, you can't cast a spell as a reaction. And if you cast a spell as a reaction, you can't then cast a spell as a bonus action (but you could cast a spell as an action, confusing I know)

Under 5.5e rules, it's much simpler—you cannot spend more than 1 spell slot to cast spells in a given turn. So if you use your reaction to cast a spell using a spell slot, you can't spend another spell slot on a spell for that turn. And if you've already spent a spell slot on a spell that turn, you can't use your reaction to spend a second spell slot.

But again, that's like an edge case. In the general case, your dad is incorrect

Quote from Angel_of_death11 >>

Me and my father seem to have a disagreement regarding the feat war cast which says  "When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature"  he says if I use this to do a spell as a reaction I can't cast a spell during my turn is this true

 

 Sage Advice disagrees with your father. 

Reaction attacks from War Caster do not take away your next rounds action. 

Quote from Davyd >>

Quote from Angel_of_death11 >>

he says if I use this to do a spell as a reaction I can't cast a spell during my turn is this true 

 

But again, that's like an edge case. In the general case, your dad is incorrect

 Sage Advice disagrees with his father and you. 

Quote from Deyric >>

Quote from Angel_of_death11 >>

Me and my father seem to have a disagreement regarding the feat war cast which says  "When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature"  he says if I use this to do a spell as a reaction I can't cast a spell during my turn is this true

 

 Sage Advice disagrees with your father. 

Reaction attacks from War Caster do not take away your next rounds action. 

 5e Sage Advice, or 5.5e Sage Advice?