So Vittorio Malvora and to some extent Lord Raith have used Outsider magic but how it is used varies differently, enhancing the formers power, and removing magic on the latter. He Who Walks Behind is the source for both of these so is there a reason for the contradictory powers? And on another note Cowl has been said to use Outsider magic but I cannot remember, where did he use it in White Night somewhere or is this just speculation I took as fact?
There are three(ish) separate effects that I think we've seen:
The first is the Ritual from BR fueled by He Who Walks Behind (and/or his Boss, w/HWWBh as the standard proxy to be sent). This is notable because anyone can use it on any given day, regardless of affiliation.
The second is Papa Wraith's unexplained immunity to Magic, which we strongly suspect but cannot specifically prove is Outsider fueled.
The third is the stuff that we saw Vitto, and later Sharkface doing. Lash described the attacks Vitto was throwing "as drawn directly from the mind of the Outsider", and then at the end of CD we see the cloud of smoke that presumably is HWWB4 flee from a meatsuit wearing semi-sentient rags or something. There is a WOJ that strongly (if somewhat jokingly) implies that Vitto was possessed by He Who Walks Before, and that vitto was HWWB4's meatsuit in CD. In the WOJ (which Im failing to find a link for at present) somebody asked Jim if Vitto was the meatsuit Sharkface was wearing, and his response was something to the effect of "oh, that would have been coo- I mean, YES! That was my intent the the time..." It was all in a very joking tone, so it's as solid a statement to some. Regardless though I think those are closer to actual possession by an outsider entity, as opposed to a sponsored magic.