We report on a simple approach for the compression of few-cycle laser pulses generated in an ultrafast laser oscillator to a duration corresponding to 1.7 cycles of near-infrared light (compression factor of 1.44) by nonlinear spectral broadening in diamond and subsequent dispersion compensation using chirped mirrors. After the spectral broadening, the pulse spectrum spans over almost an octave (580-1000 nm at the -10 dB level). The pulses are compressed by broadband-chirped mirrors and a wedge pair to a duration of 4.5 fs measured by spectral phase interferometry for direct electric-field reconstruction (SPIDER). The properties of the broadened spectrum and their modelling by numerical solution of a 1D nonlinear Schrödinger equation show that the main source of spectral broadening is self-phase modulation, whereas stimulated Raman scattering does not play a significant role.