I've read both novels (Oxygen and Nitrogen) way before the series so I was pretty excited to watch it. I hate to wait for an episode, so I deliberately waited until it was over. But meanwhile I've been reading some comments and reviews that worried me... stupid thing to do. There is absolutely nothing to worry about.
Taking into account that they merged both novels in one drama, all the staff did a good job. Obviously you can't have all and every scene from the novel, but they gave us more than enough.
Long review but I wanted it to be detailed so people who haven't read the novel can decide whether to give it a try or not.


STORY:
All what needed to be shown, has been shown. Some scene suffered minor changes, I believe due to the fact that it includes part of Nitrogen as well, but it was well delivered.
This drama tells the story of two people with their own baggage that find a supportive shoulder in each other. The romance is fast and there are no roller-coasters in their relationship. I will be more specific with the leads Solo and Gui, since it seems to be what people complain the most and I don't understand the reasons to be honest.

Solo and Gui are as inexpressive and calm as in the novel. Their relationship develops fast and doesn't have big changes. They added more drama in the series but that was something that I've expected already. Anyway the "new drama" is nothing big, so makes no difference for me.
The Solo in the drama also has his poker face, his need of attention, his possessiveness, and his hatred towards Jay and his father.
The Gui in the drama also has a kind-hearted soul, a will to help everyone, a need to take care of others, and a tendency to keep it all for himself.
People complained about how boring, dull, and inexpressive are the leads but you have to understand that SoloGui are exactly like that in the novel. That's why I recommend people to read the story before watching the drama to avoid their expectations being ruined. This is not your typical "Together With Me" or "TharnType" kind of plot. Forget about the girls causing trouble for the couple, the misunderstandings, the fights, and yada, yada, yada. I must admit that their relationship is somehow toxic too (since Solo is super possessive and Gui says yes to whatever Solo asks), but the character development that happens in the end (in both the series and the novel) makes it less worrisome. It's a calm, almost boring, romance. Expect tenderness, sweetness, and understanding (and some jealousy from Solo here and there, but that's how he is). Don't expect break ups and fights, that's nowhere to be seen.

Phu and Kao have the same owner-rabbit relationship as in the novel, and I loved every second of it. I honestly prefer Nitrogen over Oxygen, and it happened the same with the series. I like SoloGui, but PhuKao definitely stole the show for me (some "My Engineer" feelings. I liked the leads but RamKing was the best of it).
I expect a second season, this time with Nitrogen as the title and PhuKao as the leads. They clearly left their story with no closure, and the important things from Nitrogen weren't added in Oxygen. So it's quite obvious that they are planning to do a second season if everything goes well with the first one. Hopefully we will have it soon!

I don't have much to add about the third couple. The doctors, Khim, and Khem's story ISN'T part of the novel. Not sure why they added that, maybe to fill the drama with some scenes in order to not show all PhuKao's story, but you can skip their parts if you want to. The third couple has NO effect to the real story.

Then they added a couple other things that aren't part of the novel. There is a scene where Solo is drugged and almost raped. As always, thai dramas seem to forget that this type of things are a crime and the perpetrator should be punished. In the novel the problem is way less serious, but a drama needs drama.
In the series Solo has a fiancee while in the novel is introduced as the fiancee of Solo's father. Once again they needed to add some drama since SoloGui's story is basically drama-free in the novel. I would have preferred the original plot of Linda but it didn't bother me that much anyway.


ACTING:
Good job from all the cast that played a character from the novel. For some of them, Oxygen is their first official work but they did good. I'm happy with both the cast election and the execution. I felt it was a copy-paste. There were obviously some things that could have been better, but still they were able to bring to life their novel characters. I remember Houzini (the girl who translated the novels with Chesshire's permission) leaving her cute comments at the end of the chapters about how fit the actors were for the roles. They all are new faces to me so I didn't know if she was right or not back then, but now I have to agree with her. Solo, Gui, Kao and Phu were portrayed in a loyal way by the actors and that just makes the drama better.

I also loved how they didn't change the appearance of Solo's fahter. Solo was born when his father was 16/18, so he isn't even 40 yet. He is a young actor that looks young, but his father is indeed young. He also did a good job portraying his character. I'm a little disappointed with Jay. Not for the actor's job, but the fact that his appearance is completely different. Where is the blonde foreigner? The actor was clearly thai and brunette hahaha! But once again that doesn't affect the story so who cares.


MUSIC:
I liked how they added the musical parts of the novel in the drama. The chorus of the main theme gets stuck in my head every episode, and the background music that they use for the different scenes convey the vibe of it.


OVERALL & REWATCH:
If you have read the novels, then I believe you will like it. It's exactly as it's written. Some scenes missing, some details that aren't fully explained, but in a general way everything is covered. The cast did a good job something that always helps.
I would definitely rewatch it before the release of Nitrogen. Hoping for the announcement of the second season to be as soon as possible!