I reinstalled my harddrive and installed OpenBSD once again.
After that I noticed that OpenBSD now supports tmpfs, so i mounted tmp and .cache directory in ram instead:

$ cat /etc/fstab
c62268885862ee35.b none swap sw
c62268885862ee35.a / ffs rw 1 1
c62268885862ee35.g /home ffs rw,nodev,nosuid,noatime 1 2
#c62268885862ee35.d /tmp ffs rw,nodev,nosuid,noatime 1 2
c62268885862ee35.f /usr ffs rw,nodev,noatime 1 2
c62268885862ee35.e /var ffs rw,nodev,nosuid,noatime 1 2
tmpfs /tmp tmpfs -m777,rw,nodev,nosuid,noatime 0 0
tmpfs /home/johan/.cache tmpfs rw,nodev,nosuid,noatime 0 0