You are correct that a Landlord has no legal responsibility for their tenants utility accounts.
If the accounts are in the tenants name then they are nothing to do with the landlord.

Has the tenancy been ended, either by agreement or eviction, or have they done a runner?
Your post doesn’t make that clear.

If you are sure that the tenancy has ended, - if they have done a runner that doesn’t end the tenancy**, then:

You need to inform OVO that the tenant is no longer renting the property from you and that you now need a new account for the supply in your name.
Give them the meter reading(s), preferably from the date that the ex-tenant left, and preferably with a photo of them particularly if it isn't a smart meter.

You are then liable for future use after that reading.
Your ex-tenant is liable for the usage before that reading and OVO will set debt collectors to chase them for what they owe if needed*.

If you have a forwarding address for your ex-tenant then let OVO have that too*.

That's all standard practice for a landord when a tenancy ends and the tenant leaves, so there should be no problem..

*Note that if they do have to chase the ex-tenant for their debt and don't have a forwarding address for them, then to get a court order against the ex-tenant for that debt they will send letters to the property addressed either to the ex-tenant or more usually to 'The occupier'.
That can be slightly annoying/worrying but they are not doing it to chase you, or any new tenant, to pay. It's just a part of the legal process that has to be gone through before they can go to court and get a debt order against the ex-tenant. Certain letters have to be sent to the ‘Last known address’ for them.

**If they have done a runner then as said that does not end their tenancy, the tenancy is ongoing with rent arrears, and you need to take legal steps yourself to end it, ie. eviction in their absence.
Technically because they are still legally your tenants their liability for the utility bills is also ongoing,
You cannot legally take posession of the property and/or re-let the property until the existing, ongoing, tenancy has been ended.