The film opens with Parvathy (Nayanthara), a botanist, using video conferencing to emphasise the need for natural preservation in order to protect humans and other animals on the earth. Her young son Veera (Rithvik), who has breathing problems, joins her in Coimbatore. Concerned for Veera, Parvathy has placed her faith in a procedure that will take place in a kochi hospital. She leaves coimbatore with her kid in the bus and tells her brother, who is arranging the surgery with the kochi doctors, about the journey. The bus is stalled in traffic for an hour into the trip, so the driver decides to take an alternative route.

But neither he nor the other passengers were prepared for the terrifying circumstances that would arise. After a landslide, the bus loses control and becomes buried in the rubble. After realising that a major oxygen shortage is likely to happen in a few hours, Parvathy becomes worried about Veera. Can the travellers safely escape the dangerous situation? The toughest issue when creating a survival thriller is keeping the audience's interest and creating believable scenes. Despite its flaws, Viknesh mostly succeeds in turning O2 into a suspenseful thriller. Intriguing performances and edge-of-your-seat sequences are two aspects that can impress viewers for a story that takes place in such a small location.

To successfully execute the racy storyline, the director makes sure he cast the correct actors. He creates engaging personas for each passenger, which makes it simple for viewers to identify with their issues. Despite the main problem the protagonist encounters, there is tension between a father-daughter couple, a hapless driver, a terrified politician, and a ruthless cop who won't hesitate to kill others to guarantee his safety. Through naturally written moments and great performances, the minor subplots and conflicts between these characters are heightened. To put it mildly, the director's portrayal of individuals acting shamelessly selfishly when their lives are in peril is thought-provoking and frightening.

The main draw of O2 is Nayanthara as the worried mother who is willing to give up everything for her unwell son. We like the chemistry she has with Rithvik, and we feel bad for her when she loses control. While murugadoss succeeds in emotional scenes, bharath Neelakandan's performance as the adversary is deserving of applause. Both Jaffer and RNR Manohar are noticeable. Due to his charming emotions, Rithvik steals the show in some of the sequences.
In tamil cinema, the survival thriller is not a well-established subgenre. If a few flaws can be overlooked, fans of the genre will probably appreciate O2.