<i>Background and objectives</i>: Mortality of critically ill COVID-19 patients in ICU are high around the globe. There are variable reports on the outcome of invasive and non-invasive ventilation, change of oxygen saturation, and clinical characteristics in different countries and hospital set-ups. This study aimed to observe the demographic and clinical characteristics of critical COVID-19 cases, the trend of SpO<sub>2</sub> in 10-days, and the mortality outcome of oxygen therapy in a tertiary level hospital in Bangladesh. <i>Methods</i>: In this retrospective study, data obtained from 99 patients admitted in ICU with COVID-19 was confirmed by RT-PCR of the nasopharyngeal swab. The 720-bed Holy Family Red Crescent Medical College Hospital (HFRCMCH), Dhaka, Bangladesh with a 9-bed ICU facility designated as "COVID-dedicated" from May17 to September 9, 2020. Ninety-nine patients were selected for the study, divided into two groups. 39 of them were non-survivors, whereas 60 included in the survivors group. Demographic data, correlation with age groups, clinical symptoms, instrumental oxygen therapy, and mortality were collected from hospital records. Appropriate statistical analysis was done using SPSS version 26.0. <i>Results</i>: Out of 99 patients admitted in ICU with COVID-19, 72 were male and 27 were female. The mean age of the patients was 61.08 years. Most of the ICU patients were in the 60-69 years of age group and the highest mortality rates (35.89%) were observed in this age range. The presenting symptoms of the patients were shortness of breath (85.85%) was the most common symptom followed by fever (66.66%), cough (32.32%), lethargy (12.12%), and others (7.77%). The mean SpO<sub>2</sub> of their 10-days ICU stay was also variable between the two groups. A gradual increase of mean SpO<sub>2</sub> was observed in the survivors' group. Whereas, the mean SpO<sub>2</sub> level of non-survivor had ups and downs from 92% to 83% on day-10, along with the lowest level of mean SpO<sub>2</sub> (77%) was on the 7<sup>th</sup> day. <i>Conclusions</i>: With the constrain of the healthcare support system and limited ICU facilities in a low-middle income country like Bangladesh, the mortality outcome and instrumental oxygen therapy to fight the ARDS caused by COVID-19 is far challenging. The present study clearly showed the highest mortality in patients who required mechanical ventilation, whereas, almost 75% of patients survived with high flow nasal cannula (HFNC). Therefore, the experience advocates the necessity of HFNC at the earliest possible time to avoid invasive ventilation in COVID-19 patients admitted in ICU.