Oliver Cromwell had little inclination to become a great general, and indeed had no military experience at all before he felt compelled to resist the machinations of Charles, raising a troop of horse in East Anglia and join the forces of rebellion. Many minor gentry raised local forces to fight for either faction, and Cromwell's troops were no different in those early days.
After years of campaigning under Cromwell's studious and pious leadership of his the self named ironsides regiment of horses became a huge well oiled double sized unit.
Cromwell went on to command armies all over Britain, culminating in devastating and bloody conquests in Scotland and Ireland, crushing Royalist resistance wherever it was met. As Lord Protector, he became in many ways a pastiche of the 'tyrant' Charles as he himself crushed Parliament and ruled with his generals in a military Junta. After a number of years his rule became unpopular and when he died, possibly of malaria, his son Richard was foolishly offered his post, a post no one else could hope to fill, so that shortly a grateful public recalled Charles' son to the throne.
Be that as it may, Oliver Cromwell showed himself to be an able general, a superb strategist and a focus for reform in turbulent times. His military legacy was the emergence of the red-coated British army and a navy that would challenge all other nations to supremacy.