You have to change the --percent flag. It takes some experimentation. The format is three pairs of 0-100 integers, one for parameters, attention cache and hidden states respectively. The first zero is percent on GPU, the second one is percent on CPU (system RAM), and the remaining percentage will go on disk.

For disk offloading to work you may also have to specify --offload-dir.

I have opt-30B running on a 3090 with --percent 20 50 100 0 100 0, although I think those could be tweaked to be faster.