An idler-resonant femtosecond optical parametric oscillator (OPO) with near-diffraction-limited beam quality (M2 1.05) at 2.4 m is demonstrated. The OPO is synchronously pumped by a femtosecond Yb-fiber amplifier system providing 130 fs pulses with an average power of 4.5 W at 1,050 nm and delivers 90 fs pulses with maximum average powers of 600 mW for the idler and 670 mW for the signal. Tunability is demonstrated across idler (signal) wavelengths from 2.2 to 2.6 m (1.762.0 m), limited only by the OPO mirrors. As a demonstration of the utility of the source, the idler pulses are used to generate a supercontinuum from 1,600 to 3,200 nm in a silicon-core fiber.