OsH(Ph)(CO)L2 (L = p(t)Bu2Me, Ph = C6H5) is synthesized and studied by spectroscopic and DFT (B3LYP) calculations. It forms 1:1 adducts with CO or MeCN, but D2 causes reductive elimination of C6H6. Reaction with C6D6 gives exclusively OsD(C6D5)(CO)L2. Lack of conversion of OsD(C6H5)(CO)L2 to OsH(C6H4D)(CO)L2 under the same conditions indicates that zerovalent Os(η2-C6H5D)(CO)L2 is not kinetically relevant. DFT calculations confirm that an η2-arene complex is not accessible and also show that the dissociative reductive elimination of arene is a high energy process. Reaction of OsH(Ph)(CO)L2 with fluorinated arenes, HAr(F) gives benzene and OsH(Ar(F))(CO)(ptBu2Me)2 (i.e., attack on the HAr(F) C-H bond), with the fastest attack involving more heavily fluorinated arenes (but C6F6 does not react) and the attack occurring preferentially at the C-H bond ortho to fluorine. DFT calculations show that the observed isomer is thermodynamically controlled. These results, isotope-labeling results, and rate suppression and exchange involving added free phosphine lead to a mechanism where reagent arene associates with OsH(Ph)(CO)L2, but benzene loss demands preliminary dissociation of phosphine from such an adduct.