tested on openbsd 6.4 with exfat-fuse 1.2.8

mount exfat file system on openbsd

first, find your uid:

$ id -u
1000
$

then as root install exfat-fuse and mount exfat file system with your uid:

# pkg_add exfat-fuse
quirks-3.16 signed on 2018-10-12T15:26:25Z
exfat-fuse-1.2.8: ok
# mkdir -p <em>/mnt/sd1i</em>
# mount.exfat -o <em>uid=1000</em> <em>/dev/sd1i /mnt/sd1i</em>
FUSE exfat 1.2.8
#

use the file system as a regular user:

$ df /mnt/sd1i
Filesystem  512-blocks      Used     Avail Capacity  Mounted on
fusefs         7716800      1344   7710336     0%    /mnt/sd1i
$ touch /mnt/sd1i/test
$ ls -l /mnt/sd1i/test
-rwxrwxrwx  1 romanzolotarev  wheel  0 Nov  5 16:11 /mnt/sd1i/test
$

to unmount run as root:

# umount /mnt/sd1i
#