Oregano is a member of CEDEF, and therefore is a subordinate of Iemitsu Sawada. In her first appearance she make a report to Iemitsu as to the situation regarding the CEDEF and Varia. The Varia had located the base of the CEDEF, but "all 6 members" had escaped and were making there way to their current location, Vongola HQ. This implies that there are 8 or 9 members of the CEDEF in total, as Basil was in Japan and Iemitsu was not in the base when it was attacked. Next she fought along side Turmeric and Lal Mirch when they invaded Vongola HQ so Iemitsu could talk to and, if necessary, rescue the Vongola Boss. She and Turmeric found the censored Mosca plans and discovered that they sent one to the Varia. After they met back up with Iemitsu, the fake Vongola IX gave them enough information to realize that the real Vongola IX was in fact inside the Gola Mosca that was sent to the Varia. Unfortunately they were too late to stop the Cloud Guardian battle.   Source: http://reborn.wikia.com/wiki/Oregano

Nowadays, the mafia world is ruled by Italy's strongest family, the Vongola. Vongola's current head, ninth in the row, is an old man ready for retirement. But before he can hand down his position, he needs help from a professional Hitman to prepare the next generation Boss for ruling over Vongola and the mafia world itself. 
Sawada Tsunayoshi is just your ordinary teenage boy who attends Junior High and sucks at it. He is unpopular, has no friends, and is desperately in love with a girl from his class. But his life takes an extraordinary turn when Tsunayoshi meets Reborn, an infamous hitman, who the ninth boss of Vongola has sent to train Tsunayoshi for his upcoming title. 
(Source: Katekyo Hitman Reborn! FC)

Sawada Tsunayoshi is regular boy who has never excelled in sports, school or social life, thus being named "No Good Tsuna". Everyday, he leads just a normal life, living with his mother, while his father is away for work. But the thing Tsunayoshi doesn't know is that his family is actual lineage of the great Mafia Boss, the first head of Vongola, Giotto. 
Because of the Sawada history, Tsuna has been chosen to become tenth head of the Vongola family, an infamous Italian mafia family which rules the Mafia world. To become worthy of the title, Tsuna is being aided by professional hitman and infant, Reborn, who takes the role of Tsunayoshi's mafia tutor. 
(Source: Katekyo Hitman Reborn! FC)
 
Note: Chapter count includes two extra collections of short chapters published in volume 42