You can either download and install the full source package, or just compile the pre-compiled small C version of the repl/compiler and use it directly without further installation. The pre-compiled C version is suitable for quick testing and compilation of projects to C without a lengthy full Owl build. The source package takes a while to build, but the interpreter will be a bit faster. Both versions produce equal compiled code.
Precompiled small Owl: ol-0.2.1.c.gz
A quick test without installation:
$ curl https://haltp.org/files/ol-0.1.21.c.gz \
| gzip -d | cc -x c -O -o ol -
$ ./ol
You see a prompt.
> (+ 1 2)
3