Overview
An Only Representative is an EU-based party that a manufacturer outside the EU appoints to carry its REACH registration duties. Without one, every EU importer would have to register the substance itself. With one, the OR registers on behalf of the chain and the importers are covered.
The setup solves a practical problem. REACH obligations land on whoever first brings a substance into the EU, but a non-EU manufacturer cannot register directly. Appointing an OR lets that manufacturer take charge of compliance and shield its EU customers from the registration burden.
How it works in practice
The non-EU manufacturer names an OR in writing and gives it the data needed to register. The OR then handles the registration with ECHA, keeps records of quantities and customers, and keeps safety information up to date. The EU importers that buy from this manufacturer are treated as downstream users, which lightens their own duties.
The Only Representative does not remove the obligations. It moves them. One competent EU party carries the registration so that each individual importer does not have to.
Note: general educational information, not legal advice. Check the official source before relying on it.