First, the way you do it is with a program called oss2jack (http://fort2.xdas.com/~kor/oss2jack/). It works by adding 2 components to the system. One is a kernel module that can create userland character devices on demand. The other is a program that acts as a JACK client, and bridges the userland character devices (/dev/dsp?) with the JACK server.
Unfortunate, life is never simple. And the latest fusd (the userland character device kernel module) doesn't work with the latest 2.6.19 kernels.. So, here is a patch: fusd-kor-1.10-11-linux-2.6.19.patch