Data access applications in Mobile Ad Hoc Networks (MANETs) suffer from intermittent network connections and restricted power supplies. While most of the researches focus on Media Access Control (MAC) and routing layer solutions, we explore the possibility of application cooperation. In this paper, we propose COOP, a novel cooperative caching service for data access applications in MANETs. The objective is to improve data availability and access efficiency by collaborating local resources of mobile devices. COOP addresses two basic problems of cooperative caching: cache resolution and cache management, i.e. how to find the requested data efficiently and how to manage local cache to improve the capacity of cooperated caches. The simulation results show that the service significantly reduces response delay and improves data availability for data access applications in MANETs.