An ontological understanding of complexity amounts to the view that complexity is irreducible for the reasons that there is no central organising principle, and complex systems are open. In this chapter, the notion of a complex, open system is reinscribed, and further explored, in terms of a theoretical understanding of economy. The notion of economy, which denotes any constrained set of relations, was introduced into the philosophical literature by Bataille, who, in his work, distinguishes between a utilitarian or restricted economy and an excessive or general economy. In complexity terms, Bataille's central insight is the following: complexity is generated by a constrained set of relations, which gives rise to a system (the restricted economy of codified knowledge), but also to excess. The excess forms part of the system's environment (the general economy). The exact manner in which the relation between the restricted and the general economies, or the system and its environment, should be conceptualised forms the grounds for different conceptions of economy. In this chapter, five views on economy are forwarded, namely: Hegel's totalising economy, Nietzsche's and Bataille's dual economy, Derrida's aporetic economy, and Nancy's immanent economy. The implications that these positions hold for understanding both systemic openness and relationality are explored at the hand of critical analyses in order to provide a philosophical account of the ontological view of complexity.