The landlord is almost always going to be your initial point of contact. If you have any questions or issues during the tenancy then they will be best placed to help resolve these.
OpenRent doesn't currently manage any properties, so most of our landlords manage properties themselves. If you are looking to raise common issues to do with the repair of the property, its furnishings, neighbours, etc., then you the best thing to do is get in touch with your landlord, as they are the one who will be able to resolve your issue.
If there are problems you cannot resolve with your landlord, the best people to advise are Shelter who provide extensive advice on a range of problems that can arise.