Oxygen Not Included (ONI) starts you deep in an unfamiliar asteroid, in control of 3 Duplicants. The game advises you to "Start Digging." But to keep your Duplicants alive and avoid failure, you'll have to do a lot more than just dig. This guide aims to help new players through the first few cycles.
Before the actual game starts, you will be presented some settings dialogs. See Game Settings for a description of those. The recommended game mode for beginners arguably is the "No Sweat" mode and the Terra asteroid.
The first step you want to take is to start digging around to gather materials, increase your space and get access to water and other oxygen pockets. At this early stage you should try to avoid any polluted water/oxygen, poisonous gases or vacuum spaces. Although these won't kill your Duplicants (affectionately known as Dupes), they do complicate life early on. A golden rule of ONI is "Don't expand too fast too early." This includes accepting new Duplicants from the Printing Pod, not just making your base bigger! Not only will your Duplicants waste time digging out areas, your small oxygen supply will be spread out over a larger area, hastening your death by low oxygen.
Your first building should be an Outhouse, which you can find in the Plumbing menu. Because the bladders of Duplicants fill up once every cycle (twice if they have the Small Bladder trait) it is important to supply them with a toilet. If not, they'll make a mess on the floor, which creates Polluted Water. When a Duplicant steps in polluted water they will become stressed out. Luckily, polluted water can easily be mopped up. However, Duplicants must step in clean water or shower in order to clean themselves.
At this point, you can begin to think about expansion and growing your colony to survive past the first few cycles.
Buildings & Choices
It is recommended to clear a space to make Cots for your Dupes. Having a bed to sleep in decreases Stress and prevents them from getting the Sore Back debuff from sleeping on the floor. You should leave a space or two between every couple Cots for future items you might want to build (e.g., Decor). Leaving a gap between the ceiling and top of any items you build early on can be beneficial for the same reason. That way, later you can clear up this space and add items to increase the Decor, which further lowers stress. But this isn't important in the first few cycles.
What will kill most colonies is an overload of Stress. When a duplicant reaches 100% stress they will exhibit a stress response. This usually makes them incapable of carrying on with their assigned tasks. One stressed dupe can also increase the stress of others with their stress response. For example, a dupe who's a Vomiter, will of course, vomit when stressed, generating large amounts of Polluted Water. This can contaminate drinking water and spread disease, and other dupes will have to step in it to pass by or clean it up, increasing their stress levels. Left unchecked this can quickly spiral out of control, and your colony can fail quickly. Thus managing stress is one of the most important things for a colony.
One of the other most important concerns is food. Duplicants need a steady supply of calories to keep running around like headless chickens. Duplicants will burn about 1,000 kilocalories (kcal) per day, 1,500 if they have the Bottomless Stomach trait. You start with 16,000 calories worth of rations on normal difficulty, but you'll need to supplement this before long. Usually excavating nearby chambers will provide the colony with Muckroots, Meal Lice, and Bristle Berry.
Food should be your priority for now as your starting rations will only last your 3 Duplicants for a little bit over 6 cycles. You can gather a few wild Muckroot and Meal Lice growing in the air pockets around your base, but they are not a sustainable food source and are only supplemental for now. Next build a Microbe Musher, a Manual Generator, and a Battery so you can start producing Mush Bars so your colony does not starve. It is also a good idea to build a Wash Basin at the entrance(s) to your food generation/preparation area to avoid contaminating food with Food Poisoning Germs. These are unlikely to be infectious Diseases for the first several cycles, but it's better to be safe, as illness can be hard to stop after an outbreak starts.
Once you unlock the Planter Box (first item on Food branch of the Research tree), you will be able to grow your own Mealwood plants and make steady supply of Liceloaf with it. (However this may be inadvisable as Liceloaf requires a large amount of water; consider researching Meal Preparation so that you can make Pickled Meal using the Electric Grill instead.) While Mealwood's production of Meal Lice is equally slow in Planter Boxes and Farm Tiles, Farm Tiles take up less vertical space, so you will want to unlock it through the Meal Preparation research fairly quickly.
Water will be required for many things in your colony, including food production, oxygen production, and sanitation. Make your way to the nearest spot of water to allow your Duplicants to collect water and bring it where it's needed. You will need to build a manual Pitcher Pump over your water source to bottle the water to be used or moved. A Bottle Emptier will be needed if you want to move water from one reservoir to another or collect Polluted Water in a safe location for sanitation purposes. Later you will be able to move clean water around with electric pumps to make it more convenient for usage. You will also be able to filter polluted water with the Water Sieve.
Now you need a place to store all the materials you have gathered by digging around, otherwise your Duplicants will keep running around to get the materials and will gain Stress from messy surroundings. Storage Bins are easy and cheap to build, so you can build a lot of them. I suggest starting with a maximum of 2. Later on make a room to place all your Bins, as they create negative Decor and because some materials, such as Polluted Dirt, will produce Polluted Oxygen and carbon dioxide while they are in your Bins. A good course of action is to make a storage bin underwater specifically for storing slime and other infectious materials; the lack of air will kill the germs. For your Duplicants to start storing the materials in the bins you will need to click on it and choose what they should put in each Bin.
Considering the name of the game, Oxygen may sound like the most important resource, but for the first few days, it's not. The supply you start with and the pockets you connect by digging around will suffice for a while before you need to get into real oxygen production. One of the earliest and the most common ways to produce oxygen will be the Oxygen Diffuser, which requires Algae and power to work. You should already have a Manual Generator and a Tiny Battery set up by now to power up your Microbe Musher. Simply connect your Oxygen Diffuser to it with wires (ideally running them through floors and walls to avoid negative Decor effects). Algae can be found all around the world and sometimes you can hit some very big spots, but they are not renewable and producing oxygen uses a lot of it, so they won't be your source of Oxygen forever. For now one or two of the Diffusers will do the job. The next source of Oxygen to eventually use is the Electrolyzer which converts your large Water reserves into a large amount of Oxygen and a small amount of Hydrogen. Later you will be able to turn Slime into Algae, but the process is very slow and not a super effective way to make Oxygen.
Now that you have the basics covered, you can start discovering new technologies to keep your colony alive. It can be appealing to start with oxygen production researches but your colony will starve to death before running out of oxygen at this point. Start with the Basic Farming research. This will allow you to place Algae Terrariums to get rid of the Carbon Dioxide produced by digging around. However, remember to destroy the Terrariums once the CO2 levels are down as they eat up water and produce polluted water as a byproduct. You will also unlock the Planter Box which is very important to start growing Mealwood using the seeds you started with and the ones you can gather by digging up wild Mealwood plants. Place your Planter Boxes as far as possible from the other appliances as they produce heat and this will kill the plants. Power Regulation should be your next research as your Manual Generator and Tiny Battery won't last very long with all the new components you added.
Heat - the Temperate Biome
When you transition into the midgame, heat suddenly becomes a problem that will be impossible to manage if you don't take preventative steps in the early game. You start the game in the center of a Temperate Biome with comfortable temperatures, but it is much hotter outside of this bubble of safety and this heat will begin to creep in. Use the Temperature Overlay to see the temperatures in different areas. This is a good way to teach yourself how to identify the different Biomes. In addition to the heat, the neighbouring biomes will also contain dangerous Gasses and Germs. You must keep all of these out of your base!
When you dig to the edge of the Temperate Biome, it is best to seal the dangers out using Insulated Tiles and Airlocks, eventually sealing off the entire biome. Some of the dangers are already sealed away from you by veins of Abyssalite, which has near-zero heat conductivity and forms solid uninterrupted walls between certain Biomes. Learn to identify these veins, and do not breach them until you are prepared for what is on the other side.
The first few days of a colony shouldn't be too stressful. With a supply of Rations, blocks of Oxylite, and easily accessible food, it's hard to die early on. But mismanagement early on can make later days much harder, and cause your Colony to go kaput sooner than you'd like. Oxygen Not Included has lots of hidden content sprinkled around the map, so make sure to explore once you've got your base somewhat sustainable.