When the party is represented by members in the lower house of parliament, the party leader simultaneously serves as the leader of the parliamentary group of that full party representation; depending on a minimum number of seats held, Westminster-based parties typically allow for leaders to form frontbench teams of senior fellow members of the parliamentary group to serve as critics of aspects of government policy. When a party becomes the largest party not part of the Government, the party's parliamentary group forms the Official Opposition, with Official Opposition frontbench team members often forming the Official Opposition Shadow cabinet. When a party achieves enough seats in an election to form a majority, the party's frontbench becomes the Cabinet of government ministers.
When does a party form an official opposition?
When a party becomes the largest party not part of the Government