Hey TomT, It's been a while since I played with Myth, but I recall that Myth has no client for oscam to decrypt the DVB device.
When I was using Myth some time back, you had to use a loopback device that made use of the decrypt library... it used to be called sasc-ng, but that project has been replaced with ffdecsawrapper.
A quick google will show you exactly how to set it up with Myth, and that documentation will explain it far better than I can.
In a nutshell, you will have 4 DVB devices that are real (your TBS card is a quad tuner, right?), and another 4 that are decrypted and make use of oscam.
