In the bowl of an electric mixer, beat the cream cheese until light and fluffy. Whisk the lemon Jello into the hot water then slowly add it to the cream cheese. Switch to the whisk attachment and continue to beat until mixture is thoroughly combined and no lumps remain. Allow the mixture to cool to room temperature then stir in the Cool Whip and lemon zest.

Line a 9 x 13 inch pan with foil, allowing it to come up the sides so you'll have something to grab onto when the cake is finished. Start by making a layer of lemon Oreos at the bottom of the pan. Dunk each one in the milk for a few seconds then place in the pan as tightly together as possible, it's ok if there are gaps on the sides or ends. Spread 1/3 of the mousse mixture over top of the Oreos. Make another row of Oreos dipped in milk OR graham crackers (not dipped in milk). If using graham crackers, fill in the gaps by breaking the cracker up into smaller segments. Spread with another 1/3 of the mousse mixture. Finish with a final layer of Oreos or graham crackers and the last of the mousse.

Refrigerate the cake for at least 2 hours to set. When ready to serve, grab the ends of the foil and gently lift the cake from the pan onto a cutting board. Slice, plate, and garnish with crumbled or halved lemon Oreos.