OP 0 — the Ordnance Pamphlet Zero — was the master catalog of every U.S. Navy ordnance publication currently in effect. Issued by the Bureau of Ordnance, it gave ordnance officers and supply departments a single reference for identifying and tracking every Ordnance Pamphlet (OP), Ordnance Data publication (OD), Ordnance Technical Instruction (OTI), Ordnance Handling Instruction (OHI), Aviation Ordnance Modification Instruction (OMI), NAVORD Chart, and stocked War Department publication in the naval inventory. If you found a document number in a technical manual, OP 0 was how you confirmed what it was and whether it was still current.
The 1946 edition (7 May 1946) covers three parts: a numerical listing of all current publications by type (Part 1, 134 pages), a subject index organized by type of equipment (Part 2), and a checklist of required publications for ships armed with 5‑inch or smaller caliber guns (Part 3). The document is fully searchable within the viewer.