History of octanol-water partition coefficient
The most oft-cited publication is Partition Coefficients and Their Uses. A. Leo, C. Hansch, and D. Elkins. Chemical Reviews 71(6), 525-616 (1971). This manuscript was noted as a "Citation Classic" (see text here) in which A. Leo writes (in part):


  In recognition of the effort of designing
  a method of calculating log P (o:w) from
  structure, the early version of which was reported
  in this article, the Chesapeake section
  of Sigma Xi presented me with its Excellence
  in Science Award for 1980.”


Note bene: In the last article I've linked, mention is made that the phenomena was first investigated by Bertholet in 1872 and "put on a firm basis by Nernst in 1891." That said, the 1971 publication of Leo $\it{et\; al}$ is the first modern/comprehensive listing of octanol-water partition coefficients and the place where the symbology (and definition) were written.