The Collaborative International Dictionary of English v.0.48:

Compo \Com"po\, n.; pl. -pos.
   Short for Composition; -- used, esp. in England, colloq. in
   various trade applications; as :
   (a) A mortar made of sand and cement.
   (b) A carver's mixture of resin, whiting, and glue, used
       instead of plaster of Paris for ornamenting walls and
       cornices.
   (c) A composition for billiard balls.
   (d) A preparation of which printer's rollers are made.
   (e) A preparation used in currying leather.
   (f) Composition paid by a debtor.
       [Webster 1913 Suppl.]

The Jargon File (version 4.4.7, 29 Dec 2003):

compo
 n.

    [demoscene] Finnish-originated slang for ?competition?. Demo compos are
    held at a demoparty. The usual protocol is that several groups make demos
    for a compo, they are shown on a big screen, and then the party
    participants vote for the best one. Prizes (from sponsors and party
    entrance fees) are given. Standard compo formats include intro compos (4k
    or 64k demos), music compos, graphics compos, quick demo compos (build a
    demo within 4 hours for example), etc.