This paper presents the preliminary results of a study of the Knowledge Organisation System (KOS) deployed in major Digital Libraries (DLs) of the world. While traditional libraries (physical libraries) had evolved certain norms and systems, the kinds of KOS being deployed in DLs vary. Hence there is need to study in depth the Knowledge Organization (KO) tools, such as library classifications, thesauri, subject heading systems and the Knowledge representation (Ontologies) that are being deployed in DLs. This paper gives the comparative study of DLs of the world and summaries all the present scenario of KOS in these DLs. This study will help in future to build a hybrid model with different aspects of KOS deployed in DL to enhance the Information Retrieval Systems (IRS) process. Careful selecting and adopting these KOS aspects based on the suitability of the DL environment will make sure to meet the desired requirements.