Q. What does op cit. mean?

Op. cit. is an abbreviation of the Latin phrase opere citato, meaning "in the work cited." It is used in an endnote or footnote to refer the reader to a previously cited work, standing in for repetition of the full title of the work.

Op. cit. thus refers the reader to the bibliography, where the full citation of the work can be found, or to a full citation given in a previous footnote. Op. cit. should never therefore be used on its own, which would be meaningless, but most often with the author's surname, or another brief clue as to which work is referred to.

For example, given a work called The World of Salamanders (1999) by Jane Q. Smith, the style would typically be "Smith op. cit.", usually followed by a page number, to refer the reader to a previous full citation of this work (or with further clarification such as "Smith 1999, op. cit." or "Smith, World of Salamanders, op. cit.", if two sources by that author are cited).

Given names or initials are not needed unless the work cites two authors with the same surname, as the whole purpose of using op. cit. is economy of text.

For works without an individually named author, the title can be used, e.g. "CIA World Fact Book, op. cit." As usual with foreign words and phrases, op. cit. is typically given in italics. 

The Chicago Manual of Style, 16th edition, considers that op. cit. is "rightly falling into disuse," and "instead uses the short-title form" e.g. the form World of Salamanders, to use the example above. Various different styles call for other alternatives, such as a reference to the author's surname and publication year, e.g. "Smith 1999."