I think I've figured it out. Once the OUFC site is up a click on "more" brings up the log in page. Log in and you are presented with your preferences (don't know why, once you have set them it should go away!) Anyway, click save and then comes a new menu which has the option to click on "Linked accounts" That, in turn, brings up the linked account page where one can link an already existing account i.e. ticket office via fan number. 'Click here' brings up another new page which is for the on line ticketing app. This is where it goes a little wrong. Clicking on 'Return to on line shop' takes you to the on line shop but without being logged in, logging in takes you back to another page suggesting that you log in via the 'Official club sign in page' This is turn brings up your billing information for the shop. Still with me? At this point I noticed that my details are incorrect, click on 'change details' and, error message - back to ticket office.
Hopefully this is just early days and things will get sorted. I shall just ring the shop later and see if I can change my details the old fashioned way!