If your night time oxygen level was really 55%, that is very low. The first step should be to confirm the accuracy of that reported value. If that low, it would be most important to know the cause of this low oxygen level. Is it some type of chronic lung disease, such as chronic obstructive pulmonary disease (COPD), or is it due to sleep apnea, where your breathing stops during sleep sometimes for long periods of time. The most common form of sleep apnea is obstructive sleep apnea and a much less common form is central sleep apnea. I suggest that you contact your doctor and ask him/her what is the cause of your abnormal oxygen level? It is unlikely that oxygen, alone, will be the optimum treatment. You will probably need to use a breathing apparatus called continuous positive airway pressure (CPAP), while asleep and, you may also have to use oxygen in the daytime, while awake.
You should do your best to get the facts of your situation, from your doctor, and his/her recommendations for the treatment of it. If you should start using oxygen at night, before further discussion, it should be preceded by arterial blood gases (ABGs) determination and then started at a low flow, for example 2 liters per minute, with continuous monitoring of your blood oxygen levels.
If you are unable to quickly get answers to the above, you should request a second opinion from another doctor, ideally from a lung specialist, also known as a pulmonologist, with an interest in sleep medicine. Do not delay.
Good luck.