We report on the key design factors for development of Type-II “W”-lasers for O-band (1260-1360 nm) applications. We investigated the effects of InGaAs and GaAsSb Quantum Well (QW) composition and thicknesses on the emission wavelength and recombination efficiency as well as of (Al,Ga)As barriers on optimum electrical and optical confinement. Photoluminescence tests structures and full device structures were fabricated and characterised. 1.25 µm emitting lasers were demonstrated with threshold current density, Jth values of 480±10 A/cm2 at 290 K, whereas 1.3 µm lasers showed an increased Jth value of 5.5-7 kA/cm2 at 290 K.The photoluminescence test structures exhibited a similar trend of decreasing intensity with increasing wavelength. Gain measurements of the 1.3µm device demonstrate reasonably low optical losses of 10-15 cm−1 and a threshold modal gain of ≈25 cm−1.