See Bluebook rule 21.9
A basic citation to the Official Journal looks like this:
2009 O.J. (L 311) 35.
2009 : The year this issue of the O.J. was published
O.J. : Abbreviation for Official Journal
(L 311) : This parenthetical information identifies the series (the L series contains legislation) and that the issue number is 311.
35 : The pinpoint reference. On this specific page, the document cited begins.
When citing legislative acts of the Council and Commision, more than just the O.J. citation is necessary to identify the act:
Council Regulation 1355/2008, Imposing a Definitve Anti-dumping Duty and Collecting Definitively the Provisional Duty Imposed on Imports of Certain Prepared or Preserved Fruits, 2009 O.J. (L 47) 35.
Council Regulation 1308/99, Laying Down a Community Procedure for the Establishment of maximum Residue Limits of Veterinary Medicinal Products in Foodstuffs of Animal Origins, 1999 O.J. (L 156) 1.
Notice in the first example the year (2008) includes all four digits. Notice in the second example, the year (1999) is represented by the last two digits of the year. The Bluebook examples present this variation but don't provide any guidance for when the entire year as opposed to the last two digit is used. My advice is to cite the year in the same way you see it in the O.J. For practical purposes, that means that anything from the year 2000 forward will use all four digits of the year. Acts prior to 2000 will often be expressed with only the last two digits of the year.
Council Decision 2008/336, Appointing and Replacing Members of the Governing Board of the European Centre for the Development of Vocational Training, 2008 O.J. (L 115) 32.
Commission Directive 2006/75/EC, Amending Council Directive 91/414/EC to Include Dimoxystrobin as Active Substance, 2006 O.J. (L 248) 3.
When citing a Regulation, the regulation number goes first followed by a slash and the date. When citing legislation other than regulations (Directives, Decisions, Opinions, etc.) the date goes first followed by a slash and the document number. The Bluebook offers no explanation of this. It is simply the way these instruments are cited in the O.J. and the Bluebook examples reflect this practice.