The main reason for this is that then these commands can have
the same bindings in VC-Dir buffers that they have under
vc-prefix-map. 'T' is a good mnemonic for "Topic" and a
serviceable mnemonic for "outsTanding".
* lisp/vc/vc-hooks.el (vc-prefix-map): Move 'o' to 'T'.
* lisp/vc/vc-dir.el (vc-dir-mode-map): New 'T' bindings.