Entry from OED Online

[Origin uncertain; perh. an alteration of BOX-STANDARD a., after BOG n.⁴
  Differing theories of the origin of bog-standard have been proposed, but none proven. An immediate association with BOG n.¹ seems unlikely on semantic grounds. The most commonly held view is that the transition from box to bog resulted from a mishearing or misunderstanding of the earlier term.
  Others have suggested a derivation < bog-wheel former Cambridge slang for a bicycle, though ultimately also related to BOG n.⁴: see P. Beale Conc. Dict. Slang (1989) 47/2, 48/1.] 

    Ordinary, basic, standard; without extra features or modification; unexceptional or uninspired. Cf. BOX-STANDARD a.
  Although there is widespread anecdotal evidence (in personal correspondence to the O.E.D., and elsewhere) to suggest that particular association of this term (and box-standard) with motorcycles and cars dates back to the 1960s, we have yet to find earlier examples in print. In fact, as with box-standard, the O.E.D.'s earliest printed evidence relates not to motoring but to computers (although early evidence for box-standard as a noun is in engineering and mechanical contexts).
 
  1983 Austral. Personal Computer Aug. 111/1 Decryption of a 30-byte cipher block takes about five minutes, using a bog-standard Z80 running at under 2 Mhz clock rate. 1987 Graphics World Nov.-Dec. 41/4 Bog standard, cheap and cheerful card portfolios are still aplenty. 1995 Empire May 131/3 A bog-standard biography with a cheap Psycho sales gimmick, you can't help thinking Perkins deserved better. 2001 Sunday Mail (Glasgow) (Electronic ed.) 15 July, The worst thing about my Clio is that it is the bog standard version, without power steering.