Very clear. Have been doing this for at least 16 years
You need to turn on IP forwarding with the OpenBSD box (and NAT if you need it) and insure that you can route IP packets between the two end nodes (work machine and FreeBSD).... you need to insure that SSHD is running on one box (the one you want to access) and you have an SSH client on the other one.
You need to open the apppopriate SSH port on the OpenBSD box to allow things to do through... and turn this into a simple SSH client-server problem (with correct IP routing) and not a tunneling problem.
Not having IP routing (forwarding) on the OpenBSD box (your firewall) is the problem. Simply turn it on and configure away.