I‘m dual booting my Windows XP system with OpenBSD. How do I mount an NTFS file system under OpenBSD operating systems?

You need to use the mount_ntfs (or mount -t ntfs) command as root user to mount the NTFS file system that is used by MS-Windows NT or Windows 2000 or Windows XP and above. The following command will mount /dev/wd0k:
# mkdir -p /mnt/ntfs
# mount -t ntfs /dev/wd0k /mnt/ntfs
OR
# mount_ntfs /dev/wd0k /mnt/ntfs
To get volume name (in Unicode), enter:
# cat /mnt/ntfs/\$Volume:\$VOLUME_NAME
To read directory raw data:
# cat /mnt/ntfs/dir2:\$INDEX_ROOT:\$I30
See mount_ntfs command man page for more details:
$ mount mount_ntfs