The basic principle of citing Legislation is similar to books and journals.
The two main differences are that all legislation references will start with the title and will not have a listed author. You must reference legislation in full the first time you reference it but you can then use a shortened form or abbreviation. You must tell the reader you are going to abbreviate, eg. Human Rights Act 1998 (afterwards HRA 1998).
Legislation is not normally included in the Bibliography for OSCOLA. Please check with your lecturer and assignment instructions to be sure whether or not you should include legislation in the bibliography.