Edit (April 24, 2016): A Few years ago, OpenBSD developers started issuing an "installXX.fs" image which was designed to be written to a USB drive with dd(1). This makes the article below mostly obsolete.
From pretty much any unix-like operating system, you should be able to use dd to write the .fs image to the USB stick directly, like this:
sudo dd if=install59.fs of=/dev/rsd2c
(Just make sure you have the right target device specified. You don't want to overwrite the wrong drive!)
When I first installed OpenBSD to my netbook, I had tried several ways to get the ISO written to a USB stick including unetbootin, using dd to directly write the image to USB, and a few other tricks. The Googles showed me a workable yet rather complicated way to do it, but that sounded like a pain in the butt. I didn't have an external USB optical drive, but I had the hardware to hack one up. This is really how I did that first OpenBSD install:
Earlier, I had actually installed OpenBSD to a USB stick, and run the whole system directly from USB, including a swap partition, logs going to /var and the whole nine yards. OpenBSD's gratuitous logging and swap utilization killed that USB stick in a matter of months after daily use, but the thought hit me to use that same method here. By default, the bsd.rd package is installed to the root filesystem to use in a recovery situation. This is an initial ramdisk, a bare-bones userland with just enough tools to install or rescue a system. So that's what we're going to do. The TL;DR goes something like this:
- Boot a desktop from the boot CD
- Insert the USB stick you wish to use
- Choose "Install" from the CD's menu.
- Format the USB stick, and install OpenBSD to it. If it's a 4GB stick or larger, the default partitioning scheme will work great. Otherwise, make one root partition without swap. Install only the base, bsd, bsd.rd and etc packages.
- After install, re-mount the cd and copy the install files to the root partition.
- Create an etc/boot.conf file on the USB stick (mounted to /mnt) with "bsd.rd" as the first and only line.
- Shut down the system. The USB stick will now boot on most servers and netbooks lacking optical drives, and will go directly to the installer. All the OpenBSD install packages will be right there on the USB stick.
I booted up the system, and after the installer menu popped up, I plugged in my USB stick. I did this to make it obvious what its device id is. You can tell it's sd0 in this image. Adjust accordingly. At the menu, I chose "I".
You can see I'm choosing sd0 here. Again, use the right device, or you may damage important filesystems.
I just rolled with the default partition sizes here. The install files are a little over 100 megs, so they'll fit in the root partition. If you're an avid OpenBSD user, feel free to mess with the disk options here, but the defaults work well on a 4GB stick (the smallest I had laying around).
When the time comes to install sets, the only ones you really need to get an installation environment up are bsd, bsd.rd, baseXX.tgz and etcXX.tgz (where XX is the release number) - I had an official OpenBSD 5.1 install set at my desk, so I used it for this example. The numbers will change for each release, but the process works the same.
When the installer exits, it will un-mount the CD. You'll want to re-mount it to copy the install sets to USB. Also note that I set mnt/etc/boot.conf to "bsd.rd" - This forces the USB stick to boot directly to the installer ramdisk, so you'll be ready to go!
To test, I put the USB stick into my netbook and fired it up.