Here's a stupid question.
What release of OpenBSD are you using? NTFS was not added to the GENERIC kernel until OpenBSD 4.9. Prior to that release, NTFS requires a custom kernel.
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Edited to add:
And, NTFS is only available for the i386 and amd64 architectures. Here's the commit:
http://marc.info/?l=openbsd-cvs&m=128396194505847&w=2