DFT-based HSAB-related parameters predict most favorable P-versus O-complexation of parent oxaphosphirane (1) with a variety of borane reagents BR3 (2). In general, strong P-B bonds are formed, especially for R: H (3a), Cl (3d), and C6F5 (3g), in agreement with large dissociation energies, whereas O-B bonds are usually weaker. A remarkable increase in ring strain is observed upon P-complexation of phosphirane or oxaphosphiranes, especially in the case of 3g and 3d, whereas a moderate decrease occurs in the case of O-complexation for both oxirane and oxaphosphiranes. Stronger P-B bonds also correlate with larger charge transfer from the oxaphosphirane to the borane units. This in turn increases electron density at the boron center and weakens all B R bonds, thus enabling migratory insertion/ring-opening reactions in which a substituent from the borate center shifts to a ring atom; these reactions are additionally driven by release of the high ring strain of P-complexes 3.