In recent years, a large number of CertificateLess Aggregate Signature (CLAS) schemes have been proposed to overcome both the complexity of Public Key Infrastructure (PKI) certificate management and the key escrow problem. These CLAS schemes have mostly been developed for the Internet of Things (IoT). However, the current CLAS schemes require the trusted authority to manage all the devices in a network, whose number and turn-over are huge. One way to alleviate devices' management in IoT while improving access to resources is to consider a distributed architecture. In this paper, we introduce OASIS, an Organizational CertificateLess Aggregate SIgnature Scheme in IoT networks. OASIS is a hierarchical CLAS scheme that delegates the devices' management workload to multiple entities, while mitigating PKI certification and key escrow issues. We prove the security of OASIS in the random oracle model. Furthermore, the experimental results show that OASIS is well suitable for IoT distributed systems.