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 /mnt/sd1i
# mount.exfat -o uid=1000 /dev/sd1i /mnt/sd1i
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
#