Organizations increasingly split off parts and start cooperating with those parts, for instance in Shared Service Centers or by using in- or outsourcing. What is the right spot and way for finding the organization split? And on what subjects should organizations agree to cooperate effectively across the organization split? To find managerial handles for this problem, we applied action research to four large real-life case-studies in which ontology and architecture were used. This resulted in an instrument for supporting organization splitting, allying and post-merger integration, consisting of (1) organization construction rules, (2) algorithms for calculating a plausible organization splitting proposal, (3) a method for finding subjects for contracting split organizations, and (4) a real-life tested combination of all this in a way of working with (5) a known Return On Modeling Effort (ROME). Future research should make this instrument more broadly applicable, more thoroughly tested and delivering faster decision-support, and it should clarify the mutual dependency of organization splitting versus ICT splitting.