Begin by placing the Oreo cookies in a food processor and pulse until finely crushed. Melt the vegan butter and pour it over the crushed cookies. Blend until combined and press into a tart pan. Chill crust in the freezer for 15 minutes.

Blend the blackberries and blueberries until smooth, then strain through a fine mesh sieve. Mix the fruit puree with non-dairy cream cheese, sugar, and vanilla extract until smooth.

In a separate bowl, whip the non-dairy whipping cream until stiff peaks form. Gently fold the whipped cream into the berry mixture until fully blended.

Combine agar powder with non-dairy milk in a saucepan, bring to a boil while stirring. Let it simmer, then cool slightly before folding into the berry mixture.

Pour the mousse filling into the chilled crust, smooth the top, cover, and refrigerate for at least 4 hours or overnight to set.

After chilling, carefully remove the tart from the pan, warm the edges if necessary. Garnish with extra berries and serve chilled.