I can't answer specifically as to the ODP but I would always recommend that walkers use an up to date guidebook. 15 years is a long time in map making and I have no doubt that there will have been some alterations. At the very minimum open access has been here since 2005. This has the potential to be a lifesaver if you have to make a sudden exit off the hills in an emergency.
They have just reprinted Wainwrights C to C book removing a section which was dangerous, and where there was a nasty accident earlier this year. Users of old guides will not be aware of the change. ODP though, being a national trail, should be well signposted
Remember, of course, you can sell the guide on Ebay afterwards!