Inclusion Criteria:  Age greater than or equal to 18 years-old. Presence of chronic subdural haematoma (CSDH) as diagnosed radiologically either by computed tomography (CT) brain scan or magnetic resonance imaging (MRI).  Treatment of CSDH by burr-hole evacuation. Post operative pneumocephalus, as evidenced from post-operative CT Brain or MRI brain.  Negative test to SARS-nCoV-2, as evidenced by either deep throat saliva rapid test, deep throat saliva PCR test, nasopharyngeal swab real-time PCR test, or nasopharyngeal swab rapid test within seven days.  Exclusion Criteria:  Presence of pre-existing respiratory conditions such as chronic obstructive pulmonary disease (COPD) and hence not suitable for oxygen therapy.  Any pre-existing illness that renders the patient moderately or severely disabled before diagnosis with CSDH, such as a history of central nervous system infection.  CSDH arising from secondary causes, such as intracranial hypotension, thrombocytopenia, etc.  Any evidence or suspicion that there is communication between the pneumocephalus with the air cells (e.g. such as mastoid air cells) or air sinuses (e.g. frontal sinus).  Patients that need an additional procedure e.g. epidural blood patch, etc. Complications arising from the burr-hole operation or subdural drain insertion such as hemorrhage or surgical site infection requiring surgical intervention or deemed to affect the patient's long-term functional outcome.  Patients already on long-term steroid for pre-existing medical conditions. Patients who received instillation of subdural urokinase or tissue plasminogen activator via the subdural catheter(s).  Participation in other clinical trials within four weeks upon recruitment. Pregnancy or on breastfeeding. Any other reasons that the researchers consider the patients to be unsuitable.