Entry revised for OED Online

[Origin uncertain; perh. an alteration of BOX-STANDARD adj. (although this is first attested later), 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 BOX STANDARD n.
  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.
  
 
   1968 Hot Car Oct. 35/1 The brakes are bog-standard—anyway Barry says he only uses them in the paddock! 1972 Daily Mirror 15 May 21/1 She was ‘bog standard’—meaning straight from the production line without modifications. 1978 Personal Computer World 1 No. 2 30/1 They built the interface themselves, and they bought the controller bog-standard from its supplier for £3,00 or £4,000. 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.