The Publicis-owned agency beat Mediaedge:cia and the incumbents, Arena BLM and OMD.
The business was previously split between OMD UK, which handled the offline account, and the digital arm of Arena BLM, BLM Quantum, which was appointed last year to handle the online business.
Oystercatchers handled the pitch, which kicked off in February and was led by 118 118's chief executive, Gerry Murphy.
The company agreed this week to a year-long tie-up with ITV to sponsor all the films on its digital channels. The deal will see interactive content and sponsorship bumpers, created by The Brooklyn Brothers, shown during movies.
The sponsorship deal was negotiated by the company's previous incumbent OMD and ITV's commercial team.