In accordance with the present disclosure there is provided a process for preparing a hydrogenated olefin polymer by hydrogenation of a polyoctene; said polyoctene being prepared by reacting a C8-olefin feed comprising an isomeric mixture of an alpha-olefin and a non-alpha olefin monomer in a controlled manner with the help of an ionic catalyst, while controlling at least one condition of the reaction selected from the group of condition consisting of exotherm, contact time of the olefin feed with ionic catalyst and molar ratio of the ionic catalyst to the olefin feed within predetermined ranges to obtain an oligomerized mixture comprising at least one polyoctene and unreacted ionic catalyst.