Yes, all you need is `pxeboot` and `bsd.rd`. Put them in the root of your TFTP server's directory, have your DHCP server hand out `pxeboot` as the filename (and rename `bsd.rd` to `bsd` on your TFTP server) and point it towards your TFTP server, boot via PXE, and install.
You can mirror the install sets locally (on a HTTP server) for performance reasons, if you like; otherwise, you can just download them from an HTTP mirror during the installation.
I've been working on automating the whole process (fully-automated installations for bringing up new VMs) and it's so much simpler than with other operating systems.
Also, if you learn to use installboot(8) you can make your own PXE bootable images.
I would read the Makefiles in the OpenBSD source tree to see how they make their install images.
Note I'm not an OpenBSD user and have not installed it in many years so I'm not the best person to ask. I use an earlier distribution, the one that spawned OpenBSD. Similar but not the same.
