Crush the Oreo cookies in a blender until finely ground. Combine the crushed cookies with melted vegan butter and press into a 9-inch tart pan. Freeze for 15 minutes.

Blend blackberries and blueberries in a food processor until smooth. Strain if a seedless texture is preferred. In a separate bowl, mix berry puree with non-dairy cream cheese, sugar, and vanilla extract until creamy.

Whip non-dairy whipping cream in a chilled bowl until stiff peaks form (3-5 minutes). Refrigerate to keep airy.

Combine agar powder with non-dairy milk in a saucepan, bring to a gentle boil for 2 minutes. Cool slightly.

Fold agar mixture into berry puree, then fold in the whipped cream until just combined.

Pour mousse into the prepared crust, smooth the top, cover with plastic wrap, and refrigerate for at least 4 hours until set.

After setting, remove from the pan, transfer to a serving plate, decorate with fresh blackberries, and optionally drizzle with whipped coconut cream before serving.