Mix crushed Oreos, melted butter, and 1/4 teaspoon salt in a bowl. Press the mixture into the bottom and up the sides of a 9" tart pan. Freeze for about 20 minutes.

In a saucepan over medium heat, dissolve gelatin in 3 tablespoons of water, stirring frequently. Let cool slightly.

Blend 15 ounces of blackberries into a smooth puree. Strain through a sieve, reserving 1 tablespoon for topping.

Add orange juice, gelatin, and remaining salt to the blackberry puree.

Beat cream until stiff peaks form. Slowly add condensed milk, then incorporate mascarpone and blackberry mixture.

Pour filling into the Oreo crust and refrigerate for at least 1 hour.

Fold reserved blackberry puree into whipped cream.

Top the tart with whipped cream, halved blackberries, and mini Oreos if desired.