Sch 9ZD para 4
(1) A person (“P”) may register under the OSS scheme if— (a) P makes or intends to make one or more scheme supplies in the course of a business that P carries on, (b) one of the following applies— (i) P's business is established in Northern Ireland, (ii) P's business is not established in Northern Ireland or a member State but P has a fixed establishment in Northern Ireland, or (iii) P's business is not established in Northern Ireland or a member State and P does not have a fixed establishment in Northern Ireland, but P makes or intends to make scheme supplies from Northern Ireland to a member State and does not have a fixed establishment in a member State, and (c) P is not barred from registering by— (i) sub-paragraph (2), (ii) the second or third paragraph of Article 369a(2) of the VAT Directive, or (iii) any provision of the Implementing Regulation. (2) P may not be registered under the OSS scheme if they are a participant in a non-UK scheme (see para 38(1)). (3) P must register under the OSS scheme if P intends to account for VAT on scheme supplies even if P is ... registered under this Act.