The interpolation theorem of Craig is one of the most important results in formal logic. In the paper it is shown that it holds for elementary ontology of Leśniewski (LO), commonly regarded as the most comprehensive calculus of names. LO is a theoretical basis for mereology, an alternative foundation of mathematics, and can be also used as an efficient tool for direct formalisation of reasoning in natural languages. Showing that it satisfies interpolation increases its credibility of a well-behaved logical system. A cut-free analytic sequent calculus GO is used for this aim and a Maehara-style constructive proof of this result is provided.