Don't Forget Your Greens
If you are trying to get an OCD flag on a level or if you want to collect more Gooballs for the World of Goo Corporation Challenge, then you can't forget about the green Gooballs. These Gooballs are afraid of commitment (according to the game's dry humor). They can be reused in your structure. This also means that they can be freed. Once you get to the pipe in a level, you should try to deconstruct the tower. Just pull the greens off and let them climb up the structure to the pipe. It's possible to leave only one Gooball behind, since the green Gooballs can often hang on as just a free chain dangling from the pipe. It can be the difference between a normal finish and an OCD flag.
Take a Good Look
Most levels in World of Goo are fairly easy to figure out once you really take a look at them. Take a minute to scan the entire level before you commit to a single structure. Look for any unique obstacles or features that might be important to your goal. Remember, if there is a time limit for the OCD goal, you can always restart later.
Signpost are another important aspect of this search. These cryptic messages are usually good clues as to just what you are supposed to do in a level.
Be Happy When You Fail
You are going to fail a lot. That's just the truth. Just take it in stride and enjoy the spectacle of your tower's collapse. Then restart the level with the knowledge that Plan A won't work. Most of the missions past chapter 2 will practically demand that you fail once, just to figure out the level's unique dynamic. The truth is that you are usually better off if you just go with your gut and try the first crazy thing that comes to your mind. The levels are intuitive, so you won't be wrong too often.
If worse comes to worse, you can always skip the levels through the menu. You might be better prepared after you've played a bit more.
Forget OCD Mode...For Now
This is for your own good. If you are having trouble with the puzzles already, you are not going to have fun in OCD mode. A number of OCD challenges require that you complete the puzzle perfectly. Not good or excellent, perfect. If you mess up one Gooball placement, this can cost you your OCD title. Don't drive yourself crazy trying to meet the challenges. They are there for an extra boost at the end of the game. Finish all the levels and then come back to get your OCD flags.