The subjects will be randomised to receive either oxygen at the flow rate determined at the initial assessment to a maximum flow rate of 6 litres per minute, or room air. The subjects will then undergo a standard program of PR over 6 - 8 weeks and then undergo reassessment. 20 patients are required in each group to detect a 30% effect size on the 6 minute walk test (6MWT) with 80% power at the 5% significance level and 29 patients in each group to detect a 25% effect with 80% power. Therefore a minimum of 20 patients per group will be included.