Operations in object-oriented systems are often a composition of other
operations, defined across multiple objects. The aim of this paper is
to formally develop a suite of composition operators for specifying o
bject interaction through operation composition. Experience with using
Object-Z to design a range of object-oriented systems has suggested t
hat four composition operators - conjunction, parallel, choice and seq
uential composition - are necessary and, at least for commonly occurri
ng architectures, sufficient. In this paper, an abstract model of oper
ations in object-oriented systems is first developed and used as the b
asis for defining the formal semantics of these composition operators.