In-memory data sharing for grids allow location-transparent access to data stored in volatile memory. Existing Grid middlewares typ- ically support only explicit data transfer between Grid nodes. We be- lieve that Grid systems benefit from complementing traditional message- passing techniques with a data-oriented sharing technique. The latter includes automatic replica management, data consistency, and location- transparent access. As a proof of concept, we are implementing a POSIX- compatible object sharing service as part of the EU-funded XtreemOS project, which builds a Linux-based Grid operating system. In this paper we describe the software architecture of the object sharing service and design decisions including transactional consistency and peer-to-peer net- work structure. We also present preliminary evaluation results analyzing lower-bound transaction-overhead using a parallel raytracing application.