Composing together the individual atomic methods of concurrent data-structures (cds) pose multiple design and consistency challenges. In this context composition provided by transactions in software transaction memory (STM) can be handy. However, most of the STMs offer read/write primitives to access shared cds. These read/write primitives result in unnecessary aborts. Instead, semantically rich higher-level methods of the underlying cds like lookup, insert or delete (in case of hash-table or lists) aid in ignoring unimportant lower level read/write conflicts and allow better concurrency. In this paper, we adapt transaction tree model in databases to propose OSTM which enables efficient composition in cds. We extend the traditional notion of conflicts and legality to higher level methods of cds using STMs and lay down detailed correctness proof to show that it is co-opaque. We implement OSTM with concurrent closed addressed hash-table(HT-OSTM)and list (list-OSTM) which exports the higher-level operations as transaction interface. In our experiments with varying workloads and randomly generated transaction operations, HT-OSTM shows speedup of 3 to 6 times and w.r.t aborts HT-OSTMis 3 to 7 times better than ESTM and read/write based STM, respectively. Where as,list-OSTM outperforms state of the art lock-free transactional list, NOrec STM list and boosted list by 30% to 80% across all workloads and scenarios. Further,list-OSTM incurred negligible aborts in comparison to other techniques considered in the paper.