(Adds details of plan, companies declining to comment, background)
By Tatiana Bautzer
SAO PAULO, March 24 (Reuters) - A group of unidentified Oi SA bondholders has hired Moelis & Co to advise on a plan by Brazil's most indebted phone carrier to restructure part of its 55 billion reais ($15 billion) in obligations, a source with direct knowledge of the matter said on Thursday.
The group owns about $2.5 billion in Oi notes, said the source, who requested anonymity to speak freely about the situation. About 85 percent of the company's debt is in bonds.
The move would speed up Oi's efforts to stave off a potential cash crunch as debt costs surge and revenues plunge.
Oi, based in Rio de Janeiro, hired PJT Partners Inc, a former unit of Blackstone Group, as debt restructuring advisor earlier in March after Russian billionaire Mikhail Fridman's investment company LetterOne Holdings dropped a $4 billion financing offer in late February.
LetterOne's planned capital injection would have paved the way for the possible merger of Oi with Telecom Italia's local operator TIM Participações SA.
PJT has yet to present a proposal for bondholders, the source said.
Oi, Moelis and PTJ declined to comment.
Earlier on Thursday, Oi posted a $1.2 billion net loss in the fourth quarter, with debt climbing to a new record. Net revenue from Brazilian operations fell 8 percent during the worst recession in a generation.
Shares and bonds of Oi have tumbled since LetterOne abandoned its financing offer in February.
The price of Oi's 5.625 percent bonds due in June 2021 sank to 23.58 cents on the dollar on Thursday, from a high of nearly 53 cents in February.
Oi's voting shares gained 2.7 percent, but have lost 54 percent so far in 2016.
$1 = 3.70 Brazilian reais Editing by Jeffrey Benkoe and Richard Chang