If this were a power-down rather than a reboot then there would be a possibility of a misreading of a temperature sensor or incorrect fan management under ACPI. But reboots without kernel involvement indicate a hardware problem. By default, OpenBSD's kernel should drop to ddb(4) in the event of a kernel panic or other crash(8).
The memory testers can prove if there is a memory problem; unfortunately they cannot disprove the contrapositive -- they may not disclose a problem.
If memory tests after a number of hours (I prefer 8+ hours, 24+ hours when possible) do not disclose a problem, consider stress tests.
Stress testers typically work by raising CPU temps and power draw, and those can disclose a variety of hardware problems. (They also run the risk of adding heat damage if thermal controls, such as airflow, are not functioning properly.)
On OpenBSD, there are two stress testers available:
- sysutils/stress
- building OpenBSD userland from source code