"Build Fort!" is an episode of Oobi from the first season.
Oobi and Kako march to the beat of a snare drum. They decide to build a fort out of blocks. Kako wants to use circular blocks, but Oobi wants to use squares. Kako wants it to be yellow, while Oobi wants it to be blue. Kako wants to place the fort on the left side of a table, but Oobi wants it on the right. This causes a big disagreemt between them, and they each build their own individual forts instead. The boys brag about their forts after completing them. Oobi claims that his fort is strong and Kako has the same argument. Oobi proclaims that his fort is superior, which angers Kako. Oobi tries to continue speaking, but Kako cuts him off and claims that he is busy.
Later, Oobi's fort is shown with an emblem resembling a green handprint on it. Kako also has an emblem on his. Oobi declares that his fort is perfect and Kako says that his fort is "perfecto." Both boys sigh. Oobi sighs again, but in a louder tone, to prove how much better his fort is. Kako does the same. Uma walks in, wanting to play with them. The boys each want her on their side. Uma dislikes seeing them fight and leaves. Later, the boys become bored. Kako invites the viewers to his fort. Oobi invites the viewers to his fort and teaches them his secret knock, the second bar of "Shave and a Haircut". The viewers are brought back to Kako's fort, where he teaches them his secret knock, the first bar of the aforementioned tune.
The boys continue to fight as Grampu walks by, wondering why there are two forts. He comes to the conclusion that having separate forts is not very enjoyable. Oobi invites Kako over to his fort. He compliments his fort, and Oobi gives him a square-shaped cracker. Oobi walks over to Kako's fort and is given a circular cookie. The boys combine their forts to create a larger one. They teach Uma and Grampu their secret knock, the whole of "Shave and a Haircut". Oobi and Kako announce that they are friends again.
In an interview segment, Uma asks the viewers if they like building blocks. Later, Oobi and Kako are in the backyard. They play a remember game involving blocks in the shape of a circle, a square, and a triangle, which Oobi hides with a flowerpot. Afterwards, Kako disappears and is revealed to be hiding under the pot. Oobi laughs as the episode ends.