A new type of cooperative chiral order has been observed in polyisocyanates composed of a mixture of opposite enantiomers. The chiral order of the copolymer, measured by optical activity, is dominated by whichever enantiomer is in the majority. We present a quantitative theory for this cooperative chiral order, using a mapping of the random copolymer onto the random-field Ising model. Using this theory, we predict how the sharpness of the majority-rule curve can be controlled for applications in a optical switch. We also consider how this theory could be applied to chiral order in DNA.