LISBON - Oman's OQ has hired legal and financial advisers to prepare for the possible sale of its entire 12% stake in Portuguese power grid operator REN, the state-owned oil firm announced late on Tuesday.

OQ, formerly Oman Oil Company, said in a statement the advisers will "assist in the preparation of such intended sale through a private placement".

"No formal decision has been made as to the sale itself or the conditions in which it may be implemented," the statement said. "At this point no specific process has been initiated."

A sale would depend on internal approval and adequate market conditions, it added.

OQ is REN's second largest shareholder after China's State Grid, which holds a 25% stake.

REN, which owns high-voltage electricity and high-pressure natural gas transmission grids in Portugal, announced last Friday it will boost its capex by 35% over the next five years to 900 million euros ($1.1 billion). 

($1 = 0.8169 euros)

(Reporting by Sergio Goncalves; editing by Inti Landauro and Jason Neely) ((sergio.goncalves@thomsonreuters.com; +351213509204; Reuters Messaging: sergio.goncalves.reuters.com@reuters.net))