One of my players has mage slayer and they are in a tournament with another player who has shocking grasps, dissonant whispers and tashas mind whip.  I bring up these spells in particular cause mage slayer allows you to make a opportunity attack against a target that casts a spell within 5 feet of you but shocking grasp makes the target not be able to use reactions, but is a cantrip, dissonant whispers eats their reaction and uses resources  and tashas mind whip deny's them reactions. So how would you rule the interactions of each. When does the opportunity attack happen with resto the triggering event. 

*edit the player with the spells brought it up and the session is next week so they want a ruling ahead of time as to not have surprised pikachu face*

Mage Slayer gives you a reaction attack when somebody casts a spell. It doesn't say it interrupts the spell, so it doesn't. 

Therefore, the spell goes off first. If it prevents you from using reactions, then you cannot use Mage Slayer. Similarly, if it teleported them away, you also wouldn't get to use Mage Slayer.

Quote from Bendndbonanza >>

When does the opportunity attack happen with resto the triggering event.

I second what @jl8e said.

This is from the DMG

Adjudicating Reaction Timing

Typical combatants rely on the opportunity attack and the Ready action for most of their reactions in a fight. Various spells and features give a creature more reaction options, and sometimes the timing of a reaction can be difficult to adjudicate. Use this rule of thumb: follow whatever timing is specified in the reaction’s description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

And because the Mage Slayer feat doesn't specify a timing then the highlighted part applies.

Thanks.