OpenBSD rootfs

At the disk setup part in the OpenBSD installer, choose (W)hole disk MBR or whole disk (G)PT depending on your boot configuration.
Then when the installer gives you an automatic layout, choose create (C)ustom layout and then type accordingly to the output below:

sd0> a
partition: b
offset: (do not type anything, just press enter)
size: 2048M (or type any size)
FS type: swap
sd0*> a
partition: a
offset: (do not type anything, just press enter)
size: (do not type anything, just press enter)
FS type: (do not type anything, just press enter)
Mount point: /
sd0*> p (check your partition layout, then run the last two commands below)
sd0*> w
sd0> q