Oban was a type of menu for entertaining guests. Also, it later indicated the ceremony and event of entertaining guests with food.
During the Heian period, scheduled and temporary court events such as Sechie and Sechiku (seasonal festivals) had Himeii (cooked rice) piled high in a bowl as the centerpiece with side dishes such as dishes for drinking and snacks. Oban was classified into 'Tenjo no oban' and 'Shosho no oba' and this was gathered by the several designated tenjobito (high-ranking courtier allowed into the Imperial Palace) when necessary, but these were bento (packed meals)/light snacks that were considered separate from the official ceremony. Tenjo no oban' eaten by the tenjobito had a rice bowl, soup bowl, plate, small dish and chopsticks on a Oshiki (tray with folded edges) with Sai (side dishes) in Ke(dishes) inside a Oribitsu, Kashi (sweets) in a Hokai (wooden lidded container) and Heishi (sake serving cup) and hai (drinking cup) were placed on a small Oshiki. Shosho no oban' were served to officials at the Daibandokoro, Takiguchi, Mushadokoro and was simpler than the above, with a rice bowl and chopsticks on a tray, Sai in Ke inside a Oribitsu bamboo and Kashi in a Hokai were the same as 'Tenjo no oban,' but other contents differed according to the situation.
After the cloistered government period, oban started to hold a more ceremonial aspect during the Kamakura period. For example, local officials served oban as a welcome banquet for the newly appointed Kokushi and this custom of oban spread from local officials to the bushi (samurai) class. After the Kamakura bakufu (Japanese feudal government headed by a shogun) was established by MINAMOTO no Yoritomo, oban was served at important ceremonies such as Genpuku (coming of age) and Ito (moving), and the 'Saishu no oban' that was served in the beginning of the year became one of the most important ceremonies of the buke (military) government. At the Kamakura bakufu, for a few days from the first day of the year, the Hojo clan and other prominent gokenin (an immediate vassal of the shogunate) presented the Seii taishogun (literally, "great general who subdues the barbarians") with oban together with swords, good horses, and bows and arrows. At the Muromachi bakufu, there was a ceremony where prominent Shugo daimyo (Japanese territorial lord as provincial constable) family heads (toryo) served oban to the shogun and ate together, and the date to serve oban at the shogun's place was chosen for each Daimyo family. In other words, Oban was to be served on New Year's day by the kanrei (shogunal deputy) of that time, Toki clan on the second, Sasaki clan on the third, Akamatsu clan on the seventh, and Yamanaka clan on the fifteenth (However, when the Sasaki clan split into the Kyogoku and Rokkaku clans, the two clans took turns serving oban). The menu at that time was rice in a bowl with the three dishes, abalone, jellyfish, and pickled plum, with vinegar and salt on an Oshiki (tray). In addition, an event called 'Hosho', where live fish was prepared by a chef in front of the shogun was also conducted.
After the Onin War, oban was not conducted at the bakufu any more, but within the ordinary buke society, it transformed from a ceremony where vassals entertained their lord into a ceremony where the lord entertained his vassals in the beginning of the year and Sechi (seasonal) times. In the Edo bakufu, the Gosanke (the three privileged branches of the Tokugawa family) in Edo would entertain the roju (member of shogun's council of elders) and other prominent council members and hatamoto (direct retainers of the bakufu), and in a similar way, the Machibugyo (town magistrate) would entertain yoriki (police sergeant) and others at the official residence and these were considered oban.
Such customs spread to the public and the act of inviting relatives and friends at the beginning of the year was called 'Oban burumai,' 'Sechi burumai,' and this is said to be the origin of 'Oban burumai (being generous).'