The origins of regio- and stereoselectivity in the ring-opening metathesis polymerization of 3-substituted cis-cyclooctenes by monoaryloxide pyrrolide imido alkylidene (MAP) Mo- and W-based catalysts are determined at the M06-2X/SDD|6-311+G(2df,p)//M06-L/SDD|6-31G(d)|MIDI! level of density functional theory. Considering cis-cyclooctene (COE) and 3-methyl-cis-cyclooctene (3MCOE) as monomers and W(N-t-Bu)(CH-t-Bu)(OHMT)(Pyr) (OHMT = hexamethylterphenoxide, Pyr = pyrrolide) as a catalyst, all possible syn and anti combinations of alkylidene and cyclic olefin, relative to the imido ligand, are evaluated. The observed Z-selectivity for the ring-opening metathesis (ROM) of COE is due to the large size of the aryloxide ligand, which forces both the alkylidene and the incoming cyclic olefin to be syn relative to the imido ligand. As determined previously for Grubbs' second-generation catalyst (G2), breakdown of the metallacyclobutane intermediate is the rate-limiting step for cyclic olefins having ring sizes exceeding five carbon atoms. Contrary to the G2 case, however, the ring-opening of 3MCOE by MAP catalysts prefers a proximal (3-substituent closest to the metal center) over a distal (3-substituent furthest from the metal center) approach. In all calculated paths, we observe inversion of catalyst configuration after each catalytic cycle. (Chemical Equation Presented).