Crush the Oreo cookies in a blender until fine crumbs, then mix with melted vegan butter. Press into a tart pan and freeze for 15 minutes to set.

Blend blackberries and blueberries until smooth. Optionally strain to remove seeds. Mix in cream cheese, sugar, and vanilla until creamy.

Whip non-dairy cream in a chilled bowl until stiff peaks form, about 4-5 minutes. Refrigerate until ready to use.

Cook agar powder with non-dairy milk in a saucepan over medium heat, stirring until dissolved, about 2-3 minutes. Let cool.

Fold cooled agar mixture into berry filling, then fold in whipped cream gently until well combined.

Pour mousse filling into chilled crust, smooth top, cover with plastic wrap, and refrigerate for at least 4 hours or overnight.

Remove tart from pan, decorate with additional berries if desired, slice, and serve chilled.