This Data Item provides a user manual for OSMEC, a general-purpose linkage design program, which can model plane mechanisms composed of links, pivoted and sliding joints, gear trains, springs and cam-driven mechanisms and can interface with cam design program OSCAM. The program is menu driven, has extensive on-line help and can be used to design mechanisms for applications involving high speeds and large forces. OSMEC displays static and animated drawings of a mechanism and graphs and tables of positions of points, angles of links, linear and angular velocities and accelerations, driving and bearing forces and drive torque. The dimensions, masses, moments of inertia and load forces can be altered easily and quickly and the effects of these alterations are immediately analysed and displayed. Hard copy of any display can be sent to a plotter or printer, or converted to a variety of formats (including HPGL) for use in other applications. OSMEC can export a DXF file to CAD software. OSMEC allows loads to be applied to any link in a mechanism as external forces/torques or using linear/torsion springs of all types. Inertial effects may also be included. An extensive library of mechanism models representing the majority of planar linkages found in practice is included. Data Item No. 97023 describes the installation of the software and the associated hardware requirements and configuration details of the computer and associated printers and plotters.