# fdformat /dev/fd0c
Format 1440K floppy `/dev/fd0c'? (y/n): y

When you type y, fdformat(8) will start running a low-level
format to prepare the disk to receive a files system. Low-
level formatting is the slowest part of making a floppy usable.
Once you've formatted the disk, you can decide to put either a
FFS or MS-DOS filesystem on the floppy.

MS-DOS File Systems

To swap data between a Windows machine and your OpenBSD box,
format your floppy with the MS-DOS file system. The OpenBSD
program newfs_msdos(8) provides this functionality.

------------------------------------------------------------
# newfs_msdos /dev/rfd0c
/dev/rfd0c: 2840 sectors in 355 FAT12 clusters (4096 bytes/cluster)
bps=512 spc=8 res=1 nft=2 rde=512 sec=2880 mid=0xf0 spf=2 spt=18
hid=0
#
------------------------------------------------------------

FFS File System

FFS file systems need a valid disklabel on every disk, even something
as simple as a floppy. disklabel(8) can grab predefined disklabels from
/etc/disktab and copy them to a disk, which simplifies the process
considerably. While disklabel(8) can also create partition information
or mark a disk as bootable, this is all overkill for a floppy disk.
You can do all the required labeling by just running:

------------------------------------------------------------
# disklabel (1) -w (2) /dev/rfd0c (3) floppy
------------------------------------------------------------ 

This 1st "-w" option tells disklabel(8) to write to the raw disk device
(2) /dev/rfd0c, using the (3) "floppy" label from /etc/disktab.
Now that you have a label, you can create a file system with newfs(8).

------------------------------------------------------------
# newfs /dev/rfd0c
/dev/rfd0c:     2880 sectors in 80 cylinders of 2 tracks, 18 sectors
        1.4MB in 5 cyl groups (16 c/g, 0.28MB/g, 64 i/g)
super-block backups (for fsck -b #) at:
 32, 640, 1184, 1792, 2336,
#
------------------------------------------------------------

That looks much more interesting than the MS-DOS file system-creation
output, doesn't it? FFS is a more complex file system than any variant
of FAT. The various MS-DOS file systems are more interchangeable between
machines, however, being something of a lowest common denominator these
days. You need to decide what best suits your needs.