Tested on OpenBSD 6.3.

Mount disk with... mount(1)

Only root can mount(8) file systems on OpenBSD, a regular user should run mount(8) via doas(1).

Plug in a USB drive and check system messages:

# dmesg
sd1 at scsibus2 targ 1 lun 0: <Vendor, Model, 1.26>
SCSI3 0/direct removable serial.12345678901234568789
sd1: 7633MB, 512 bytes/sector, 15633408 sectors
#

Check partitions:

# disklabel sd1
...
      size     offset  fstype [fsize bsize   cpg]
a:    736256       1024  4.2BSD   2048 16384 16142
c:  15633408          0  unused
i:       960         64   MSDOS
#

To mount a partition, for example, a:), use /dev/sd1a device.

Create a mount point directory, for example, /mnt/usb-drive, and mount the drive:

# mkdir -p /mnt/usb-drive
# mount /dev/sd1a /mnt/usb-drive
# ls /mnt/usb-drive
...
#

It's mounted.

Before disconnecting the drive from the USB port, unmount it. Leave mount point directory and then use it as an argument for unmount(8).

# cd
# umount /mnt/usb-drive
#

Or you can address your device directly:

# umount /dev/sd1a
#