Not familiar with Osmos, but isn't everything that moves in agar.io a player?
If you're looking for some random movement, you could use the bullet behavior and have a condition every x seconds to set angle to random(360).
For some AI that heads towards the player when close, you can use the condition that checks distance between player and object, and action that set object angle towards player.
If you add another condition to check if the player or object is "larger", you can set up the inverse action/angle to run away from the player as well.