An early 1980s Animated Series by Hanna Barbera, based on a group of animal characters from Hallmark greeting cards. In the Shirt Tales cartoon, the animal heroes had their base of operations in an old oak tree.
Overt Operative: Possible subversion. No one is supposed to know who the Shirt Tales really are, let alone that they are five (later six) talking animals. The mere fact alone that they are talking animals is the perfect cover since most humans would assume that the Shirt Tales were also humans. So, they aren't as "overt" as the trope would imply.