Game will not start at all.

The game was unable to connect to Steam and so it was closed. Previously I've noted an outdated Steam client could cause this but you mentioned it working recently. 

Could you share your Help -> About Steam? Here's what mine looks like.

Try uninstalling the game and removing this folder before reinstalling.

C:\Program Files (x86)\Steam\steamapps\common\OxygenNotIncluded

 

Here it is:

I tried your suggestion. Uninstalled and installed again, removed the folder.

I also removed following additional folders:
Documents\Klei
AppData\Local\Klei
AppData\Roaming\Klei

I also tried running it directly from the application folder. It says that the application is already running (It is not running in background)

It could be that your antivirus, security, or firewall is interfering with the game. Try temporarily disabling them to see if that's the case and check their logs.

Previously players have found success in resolving this issue by reinstalling Steam. You can do this without losing your installed games if you move your steamapps folder first.

Ok, it might seem like the issue with steam itself. 

The issue is the same with one other game, as i discovered today. The other game is completely not related to your studio and seem to have the same issue, so far.

Imediatelly after the PC restart it works but after some time, or when i play other games, it just not.I will try the antivirus and other stuff, but the issue seems to be not related to your game as i initially tought it would be. Strange thing is, it started to show after the latest stable branch update and not before.

Anyways, thank you for support and your really quick reaction time. You can close the issue as it is most propably not related to your game. I would do it myself but i do not know how or if i can.

If you figure out what's going on please let me know so I can help others in the future. You can try contacting Steam support about issues with the Steam Client. They have a troubleshooting guide too.