O’Brien was reported missing on 24 January after his family became concerned for his well-being when he failed to return after stating he was going into the bush.
Police said in a statement that O’Brien was known as an experienced bushman who frequented the tracks across the county.
“Unfortunately it was not known where he would have entered the bush,” the statement said.
Over the past week, police have extensively searched the West Coast for O’Brien’s car, in an effort to narrow the search area.
It was discovered yesterday in an area not clearly visible on Bell Hill Road, Moana.
Search teams have been deployed to the area and resumed searching today.
Searchers include Police Search and Rescue, LandSAR volunteers including a search dog team, Kotuku Surf Lifesaving and a helicopter company.
Anyone with information about O’Brien and his possible whereabouts or plans was asked to contact police on 105 and quoting file number 230124/3168.