Johnson told Echosport: “This is great news for myself and the club.
“Roy is a top professional and the type of player I’m trying to attract to the club which was clear to see when he was here on loan over Christmas and the New Year.
“We offered him a position with us as player-coach on an 18-month contract and Roy seems keen on the idea.
“There are a few things he needs to settle at the Yeovil end of things but I’m hopeful everything will be sorted over the weekend.
“It may even be that, if the stops are pulled out, Roy will be in our side at Basingstoke tomorrow.
“As a player, he is as fit now as he ever has been and his experience and know-how will be also be invaluable regarding the coaching.
“Bringing Roy to the club on a long-term deal is a great bit of business for the club and this signing shows that myself and the board are doing all we can to take us forward.”
Johnson said O’Brien’s addition to the coaching staff would not affect the current roles filled by Gary Borthwick, John Waldock and Dave Kiteley.
“They are all still part of my plans and Roy’s arrival doesn’t change anything,” he said.
“Roy is here primarily as a player but he also wants to increase his involvement in coaching which is what we’ve offered him.”
Johnson revealed he held talks with wantaway defender Steve Tully This came after the former Torquay United defender claimed Weymouth were not good enough to win promotion this season.
“I spoke with Steve and explained I was disappointed by some of the things he said,” he said.
“We’ve accepted his transfer request, will see what offers there are and go from there.
“He’ll be in the squad to travel to Basingstoke tomorrow but no decision has been made on whether he plays.
“I’m not going to try and gain the respect of one and risk losing respect of 15 other players.”
Talks with Tully capped another busy night for Johnson who officially acquired Yeovil hot prospect Charlie Welch and himself signed a new sixth-month rolling contract with the club.
“I’m delighted the club offered me a new contract because it shows the faith they have in me.
“I was working under a verbal agreement before but this rolling contract gives me that extra bit of stability and allows me to start planning a bit more longer-term.”
Striker Lee Phillips is ruled out of tomorrow’s trip to Camrose after failing to recover from his groin strain.
His absence means former Anderlecht youngster Ludovic Cadima, who is on a deal until the end of this season, is likely to partner Kirk Jackson up front.
“It was disappointing to lose against Thurrock last week but I saw enough in that one to indicate we’re capable of beating Basingstoke tomorrow,” said Johnson.
“It’s a big game for us as they are aiming for the play-offs like us and three points will be a big boost.”