I have no knowledge of OVO but, as a dual fuel customer, I did switch from British Gas to Scottish Power some years ago. The switch happened without hassle and we saved a lot of money. Scottish Power were not the cheapest but, at the time, they had a good customer rating. I have had no major problems with my new provider.
I keep my meter readings up-to-date and keep an eye on my credit balance to make sure the company's computer is not building the credit balance too high. I can adjust it on-line if it does. I have taken fixed price contracts which have served me well but, if I want to, I think I can switch out without penalty.