No one here can access your account @martin78 as it’s a public forum. 
Do you have your first bill to check how that was reported? There should have been a starting reading for your energy which would correspond with the last reading from the previous tenant unless that was a problem at the beginning 

I got the first bill from November 2023 (I joined in September 2023). The opening gas reading is correct and there is no debt displayed there. I remember to receive letter for the previous tenant, he had different account number. I tried to solve it with OVO many times, but I was always told to ignore that. I don’t know the name of the previous tenant or any details.

So since November 2023 when you got that first bill, until now (November 2024) have you had any other bills from OVO or paid anything to OVO?
(If there were bills then were they based on actual meter readings or on estimates?)

What date was it that you left OVO?

PS, “Migrated Balance” sounds like a debit balance that has been transferred (migrated) to your new supplier.
Are OVO actually asking you to pay anything now?

I got the monthly bills in my email box. They were often based on Estimated reading, so I asked for new Statements based on Actual readings. Then I paid the monthly statements same day. I joined on 21/09/2023, got the first bill in November 2023 and left on 04/10/2024.

It looks that they incorrectly ignored the final ACTUAL reading which was lower than the ESTIMATED reading used in the next to last bill. I feel sorry that OVO is unable to spot that on first place and I must wait now for about 2 weeks to complete the investigation and get a new final statement. There was a reason why I decided to leave OVO…

I suspect that whoever has moved into the place that you have just left may have given a different ‘moving in’ meter reading to your ‘moving out’ reading.

It sounds like you have already contacted OVO to query this?

I suspect that whoever has moved into the place that you have just left may have given a different ‘moving in’ meter reading to your ‘moving out’ reading.

I don’t think so. I think that the problem was caused by OVO during the recent migration. They migrated the estimated reading and they did not adjust for the “actual to estimate” difference in the final statement.