Oregano is a member of CEDEF, and therefore is a subordinate of Iemitsu Sawada. In her first appearance she makes 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 theire 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 alongside 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.
As with most members of CEDEF revealed thus far, Oregano is named after a plant used in cooking: the Oregano plant, unsurprisingly.