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.