The default OpenBSD partition layout uses a pre-defined template. If you have a disk more than 356 GB you will have unused space with the default layout (346 GB before 6.4).
It’s possible to create a new partition to use that space if you did not modify the default layout at installation. You only need to start disklabel with flag -E* and type a to add a partition, default will use all remaining space for the partition.
# disklabel -E sd0 Label editor (enter '?' for help at any prompt) > a partition: [m] offset:  size:  FS type: [4.2BSD] > w > q No label changes.
The new partition here is m. We can format it with:
# newfs /dev/rsd0m
Then, you should add it to your /etc/fstab, for that, use the same uuid as for other partitions, it would look something like 52fdd1ce48744600
52fdd1ce48744600.e /data ffs rw,nodev,nosuid 1 2
It will be auto mounted at boot, you only need to create the folder /data. Now you can do
# mkdir /data # mount /data
and /data is usable right now.