The Collaborative International Dictionary of English v.0.48:

Oncost \On"cost`\, n. (Accounting)
   In cost accounting, expenditure which is involved in the
   process of manufacture or the performance of work and which
   cannot be charged directly to any particular article
   manufactured or work done (as where different kinds of goods
   are produced), but must be allocated so that each kind of
   goods or work shall bear its proper share. [Brit.]
   [Webster 1913 Suppl.]