OCD or Obsessive Completion Distinction are optional achievements available in World of Goo. OCD is not relevant to completing the game (unless trying to complete everything in the game). Getting OCD requires one of three things: getting a large amount of Goo Balls (usually more than double the normal requirement), completing the level within a certain time limit, or completing the level with a minimal amount of moves. The OCD requirement for a level can be checked by opening the menu and then selecting the OCD option.
Once a level is completed with OCD it will be flagged on the chapter screen with a black flag (red in Chapter 4).
Most commonly, OCD will require that the player collects a large amount of Goo Balls. This usually requires very precise strategy and using Goos leg extensions to their full length. A lot of OCDs also require different approach to the level and the use of special techniques like pipe hanging or Goo throwing to achieve or even surpass OCD.
Some OCD requirements force the player to beat a level in a certain time limit. This usually will require the player to practice beating a level quickly, without thinking about what move comes next.
Some levels OCD will require that the player complete the level in a certain amount of moves. Moves are measured by how many times a Goo Ball is attached to a structure. Launching a Launch Goo or an Infected Goo as well as grabbing a Block Goo are counted as moves.