In a two-stroke opposed-piston engine, a ported cylinder with a pair of opposed pistons is equipped with a decompression port including a valve and a passage with an opening through the cylinder wall that is located between the cylinder's intake and exhaust ports. The decompression port enables release of compressed air from the cylinder after the intake and exhaust ports are closed. The valve is opened to permit compressed air to be released from the cylinder through the passage, and closed to retain compressed air in the cylinder. Engine braking is supported by release of compressed air through the decompression port into an exhaust channel when the pistons are at or near top dead center positions as the cycle transitions from the intake/compression stroke to the power/exhaust stroke. Compression release from the cylinder after intake and exhaust port closure can also support other engine operations.