We have just finished the ODP in 12 days (with only 2 hours rain en-route!)
A word of advice: The 2004 edition of "Offa's Dyke Path" by Keith Carter states that the bridge at Bryndrinog between Newcastle-on-Clun and Chirk has been condemed as being unsafe and that a diversion via Newcasle is necessary thus adding an extra 1 hour to your walking time (and this on the "Switchback" section".
This is untue, the bridge is "safe and sound" - it looks like a newish bridge to me.