This post is from the now defunct website “A GameDev Plays…”, copied here for posterity
Ever wondered how things would be if you were just a single cell, living a simple cell life? Floating around, absorbing your smaller neighbours, desperately avoiding being absorbed by your larger neighbours? Well wonder no more, as that is exactly the theme behind Osmos, a vaguely physics based puzzler, that aims to be relaxing, but often ends up exceedingly frustrating.
In Osmos, you play as the aforementioned cell. Movement is created by ejecting part of yourself and as a result moving in the opposite direction due to conservation of momentum. Understanding momentum is absolutely vital in this game, as your cell shrinks (due to ejected mass for movement) or slows, it becomes more mobile and vice versa. Ejecting material is just a matter of clicking on your cell where you want it to emerge. Clicking repeatedly results in more material being ejected and thus a greater change in velocity. That, plus keys to zoom in/out and speed-up/slow-down time is all that is required to control the game - simple and well-designed.
The challenge in most levels is to grow. Each level contains various other cells and there is a simple rule for how interaction occurs. If you touch another cell and you are larger, then the other cell is slowly absorbed, its mass becoming part of yours. If you are smaller, then you are absorbed. If your cell is totally absorbed then the game is over - time to restart. A nice touch in Osmos is the use of colour. If another cell is bigger than yours it is coloured red, if it is smaller then it is blue. So the general rule is: move towards blue and avoid red. The player quickly knows exactly how to succeed, but of course the challenge lies in actually managing to succeed.
This game is definitely challenging - verging on frustrating. Osmos may be easy to understand and may have the style of a relaxing game, but many of the levels can be frantic at the start as the player desperately tries to get big quickly. If the player has not reached a good size after the first few seconds, it is usually best just to restart. On the Steam version there are 42 levels across 3 groupings and the feel between them is completely different. There are: ambient levels which are the closest to puzzles; force attractor levels, where everything rotates around a central object; and artificial life levels where you are up against AI opponents. Interestingly there are more levels and groupings available on the mobile version of the game (which I have not played). I guess that is the better selling platform. The levels are randomly generated, and the RNG gods can easily condemn you to a quick death. However, the player can retry the same level configuration or generate a new one on demand.
While this is a pretty, enjoyable and well designed game, the change in style can be jarring. The game suggests it is all about a calm casual experience. The music, graphics, speed and even the marketing copy all promote relaxing gameplay. After the initial scramble for size, the game often slows down to a survival of the fattest. Here the game provides the promised tranquillity, as your cell slowly bounces around (even with sped up time) and the music plays. Although until that point, Osmos is anything but relaxing. It is unlikely to be an attempt at modelling cellular life, so instead it feels a strange juxtaposition. Considering the sharp drop off in players reaching certain Steam achievements many people find the game too hard (or perhaps they don’t appreciate the style change) - 68% of players complete the first 5 levels; only 14% complete the following 10. Potential purchasers should be aware of what they are getting - a good physics puzzler which can sometimes be frantic and sometime slow - often in the same level.