Best answer by Mw2870
I had a similar experience when I switched to OVO from NPower.
My Electricity and Gas were set to switch on exactly the same day, but subsequently when I tried to add meter readings on the website I was told that one of them had not switched.
I called customer service and was told that the message on the website is misleading, basically it means that they are waiting for the old supplier to confirm the opening reading, which you will have supplied around the date of the switch.
Why this happens at different times when you are switching from the same supplier, I have no idea! Maybe different computer systems, the joys of the energy market in the UK!
Anyway I found that a couple of weeks later I could add meter readings as normal. Hopefully you will have the same experience and that OVO can update the message on the website to make it more explanatory!