ossp works on 11.04. I downloaded the modified libfuse from this page: http://mirror.leaseweb.com/kernel/people/tj/ossp/ and ossp from this: https://sourceforge.net/projects/osspd/
once libfuse was compiled and installed I had to modprobe cuse.
chmod 777 /dev/cuse (i'm sure theres better permissions but I'm lazy).
copy the ossp binaries to /usr/bin
run ossp -v -f
chmod 777 /dev/dsp
chmod 777 /dev/mixer
chmod 777 /dev/adsp
I had perfect sound output from the application aqualung when started with aqualung --output oss.
It appears that mmap is not currently working though. On this page: http://osspd.git.sourceforge.net/git/gitweb.cgi?p=osspd/osspd;a=log;h=HEAD it states that 7 months ago the fuse mmap api changed so mmap is currently disabled. There was another release of osspd (the one i'm using now) 2 months ago but that does not have working mmap, so games based on quake3 engine (and anything else using mmap) will still not work.
The output from ossp opens a connection and closes connection as expected with aqualung, however as soon as an mmap connection tries to open, it closes.