Cloud storage is widely used to share and collaborate on files over the Internet; consistency maintenance of replicated files in the face of concurrency is a major challenge. In this paper, we present a novel CSOT (Cloud Storage Operational Transformation) solution to support real-time file synchronization in front-end cloud storage and achieve consistent and desirable concurrent operation combined-effects that cannot be fully achieved by any existing cloud storage systems. We have formally verified algorithmic correctness of CSOT, built a proof-of-concept implementation, and experimentally compared results produced by CSOT and three industrial cloud storage systems, Microsoft OneDrive, Google Drive, and Dropbox, under the same collection of concurrent operation testing cases. CSOT is the first to extend OT consistency maintenance capability to shared workspaces in cloud storage and contributes to advancement of cloud-based collaboration technologies.