1. A method of operating an opposed-piston engine, comprising: generating exhaust gas in at least one ported cylinder of the engine; transporting exhaust gas from an exhaust port of the ported cylinder through an exhaust channel; recirculating a portion of the exhaust gas from the exhaust channel through an EGR loop; pressurizing fresh air; mixing recirculated exhaust gas from the EGR loop with the pressurized fresh air to form charge air; pressurizing the charge air with a supercharger; providing the pressurized charge air to an intake port of the ported cylinder; determining a value of a trapped air handling parameter in response to an engine operating state; and, adjusting, based on the determined value, charge air flow to the intake port.