Quoting Steve Shockley <steve.shockley@shockley.net>:

quoted text
> stan wrote:
>> Is it possible to do this on the one disk. I do have enough space, my
>> concern is about portions. If it is possible can anyone give me an idea how
>> best to approach this? Or a pointer to some docs?
>
> I've done what you mention using Acronis Disk Director or Partition  
> Magic, but they're not Free in any sense.  Resizing partitions is  
> handy when multibooting, but I'm not familiar with a partition  
> resizer that works with OpenBSD partitions.
>
> If you have a laptop, it may be easier to just swap disks.
>
>

You could also check this link  
http://readlist.com/lists/openbsd.org/misc/2/11903.html it has worked  
for me in the past.