It works!
I'm now running ossv4 with my STX by compiling the above code and it sounds beautiful

Only problem so far is that I'm hearing a static noise in the background, and since I'm no ossv4 expert I'm not really sure of what could cause it.
The static is present both with and without vmix0 enabled, and have disabled all microphone stuff. It is only when playing sound the static is present.
Maby tomorrow I can test with another sound card to see if it is an issue with the STX-driver or if something is wrong with the OSS itself.
And a little note: If i when testing this code discover something, who should I send what I've found and where?