The "simple" way is an extra NIC in your OpenBSD router and a second AP, which is probably $50 outlay. But if you don't want to spend any money, or are doing this more to learn than anything else then...
DD-WRT support VLANs (virtual LANs) and VWLANs (virtual wireless LANs), and can have separate DHCP on the different VLANs. To set the DHCP on the V(W)LANs and the ipfilter rules for the traffic allowed between the VLANs and the WAN (or between the different VLANs) you need to use the "startup commands" rather than doing it all through the admin WebUI. See e.g. http://www.dd-wrt.com/wiki/index.php/Multiple_WLANs, http://www.dd-wrt.com/wiki/index.php/VLAN_Support.
DD-WRT also supports 802.1q VLAN trunking, which allows traffic from multiple VLANs to share a single ethernet port/cable/NIC -- although the support for this depends on the specific chipset/board/device: see the previous link. OpenWRT will have similar features. Most consumer/SOHO routers won't expose such features in their stock firmware, of course.
I assume you want to keep OpenBSD as your gateway router. I don't know OpenBSD but it almost certainly supports VLANs and 802.1q VLAN trunking. So you could use DD-WRT/OpenWRT on your AP/wireless bridge and set up a VWLAN for the "guest" wifi, bridged to a corresponding VLAN, plus the normal WLAN bridged to the normal VLAN, with 802.1q tagging on the "LAN/WAN" port (which is in both VLANs) that connects the AP to the OpenBSD router. Configure OpenBSD with the same VLANs/tags and set up pf rules as needed.
Each VLAN will be in a different subnet (e.g. 192.168.1.0/24 and 192.168.2.0/24) but each will be only single-NATed to the WAN. You'll need to set up DHCP on each VLAN's subnet, presumably on OpenBSD (or you could use the AP to provide DHCP for the "guest" subnet, I guess).
Just a SMoC (simple matter of configuration)...
With either solution you'll want some QoS/throttling too, probably on your OpenBSD box. (You could do throttling on the DD-WRT WAP -- DD-WRT has some QoS functionality -- but it wouldn't know about wired traffic on the internal network, so would be limited in the scope of QoS/traffic prioritization it could enforce.)