Pick over crabmeat, removing any shell or cartilage.
In a bowl, combine crabmeat, mayo, juice of 1/2 lemon, onion, garlic, parsley, oregano, salt & pepper. Mix lightly.
Cut each avocado in half, peel and remove the pit. Cut each half into 6 to 8 wedges, squeezing remaining 1/2 lemon over slices to prevent discoloration.
Place crabmeat in center of large serving platter. Arrange avocado and tomato wedges alternately around crabmeat.
Sprinkle wedges lightly with salt.
Serve.