I have an iMac G3. It's old. I like it. More than once, I screwed up the OS and had to install again.
You have more than one way to do it. You can boot from a CD-ROM, but mine is broken, as most of the CD-ROMs drives on dated machines. Or boot from the network, but you have to setup the environment. Or boot from an USB flash drive, which I didn't think it was possible until I tried a few days ago.
There are some discussions about how to correctly create a bootable USB install media for OpenBSD. Recently, Chris Cappuccio announced the availability of images ready to be copied to an USB flash drive, but they are for i386 and amd64 ports only.
One way which is very debatable on how to do this is to dd the ISO image directly to the USB flash drive. Not because it doesn't work, but because it may not work on all systems. A .iso, more specifically an ISO 9660 .iso file, is a filesystem suited for optical discs, not USB flash drives. Some firmwares (IBM PC BIOS, Open Firmware) will see them as a disk, but you can't be so sure about this. However, this is not the only problem. If you want to dig about the concerns raised on the mailing lists, you can check the archives here and here.
What I'm putting up here is not what you should do, but instead what have worked for me. It's a Reminder For Myself. OpenBSD team is very rigorous about the project documentation, and what's stated at the INSTALLATION NOTES for OpenBSD/macppc 5.6 should be applicable to all supported machines, not just my case as I'm doing here. So, if you read it here, or any other blog for that matter, don't ask it on the mailing lists. Refer to the official FAQ. Refer to the man pages. If you don't find what you're looking for on these references, then yes, ask on one of the mailing lists.
At the time of this writing, the latest version of OpenBSD is 5.6. So, download the cd56.iso from your preferred mirror:
$ ftp ftp://ftp.openbsd.org/pub/OpenBSD/5.6/macppc/cd56.iso
dd'it to your USB flash drive:
# dd if=cd56.iso of=/dev/sd1c bs=4k
To check, in Open Firmware, at some point you'll see something like this:
0 > dev / ls ... ff909748: /pci@f2000000 ... ff926b98: /usb@18 ff947668: /disk@0 ff92e820: /usb@19 ff947288: /keyboard@1 ... 0 >
If you remove the USB install media, you'll note that this turns out to something like this:
0 > dev / ls ... ff909748: /pci@f2000000 ... ff926b98: /usb@18 ff92e820: /usb@19 ff947288: /keyboard@1 ... 0 >
Notice that the disk just below /usb@18 gone. Of course, this can vary depending on the model of your machine, the Open Firmware version, etc, but nonetheless it will be very similar. And you don't need the whole path to indicate from where to boot. You can use the devalias command for this:
0 > devalias ... usb0 /pci@2f000000/usb@18 ... 0 >
0 > boot usb0/disk@1:,\ofwboot OpenBSD/macppc boot> boot 5.6/macppc/bsd.rd
Also note that cd56.iso doesn't have a bsd.rd right at the root, so you need to specify the path to it. Now install OpenBSD/macppc almost as equal to any platform.