Start by placing Oreo cookies into a food processor and blending until they turn into fine crumbs. Melt vegan butter and combine it with the cookie crumbs until fully mixed. Press this mixture firmly into the bottom and sides of a 9-inch tart pan. Chill in the freezer for 15 minutes until the crust is set and firm.

In a blender, combine fresh blackberries and blueberries, blending until you achieve a smooth puree. Transfer the berry mixture into a mixing bowl, then add non-dairy cream cheese, sugar, and vanilla extract. Stir the ingredients together until fully incorporated.

In a separate mixing bowl, pour in non-dairy whipping cream. Using an electric mixer, whip the cream on medium speed until stiff peaks form, about 3-5 minutes.

In a small saucepan, dissolve agar powder in non-dairy milk and bring it to a gentle boil. Once boiling, remove from heat and let it cool slightly. Fold this agar mixture into the berry puree along with the whipped cream until fully blended.

Carefully pour the berry mousse filling into the chilled Oreo crust, smoothing the top with a spatula for an even finish. Cover the tart with plastic wrap and refrigerate for at least 4 hours or overnight.

After the tart has set, remove it from the tart pan and decorate with additional fresh berries before slicing and serving chilled.