Mercer linked up with his new team for the first time on Wednesday ahead of the start of the Guinness PRO14 this weekend.
The Ospreys face Zebre in Swansea on Saturday, with Tandy delighted to add the “quality and experience” Mercer will bring to his squad. The 27-year-old flanker captained Bath last season.
“It’s no secret that we’ve been a bit light in back-row numbers” said Tandy.
“To pick up a player of Guy Mercer’s quality and experience is excellent news for us and will give everybody a boost ahead of the PRO14 kicking off this weekend.
“He’s really keen to come down here and get started. He’s pushing himself in the right direction and competing for selection. He will provide another quality option for us.
“He is a quality player and person, someone who is a natural leader, which will stand us in good stead right the way through the season. You don’t become club captain at Bath if you don’t possess certain characteristics and we are looking forward to seeing him bring those credentials into our environment.”
Mercer has made more than 120 appearances for Bath after coming through the academy at the Aviva Premiership side. Tandy believes the forward can swiftly establish himself as a senior figure at the Liberty.
“He will really add to our depth,” the Ospreys head coach continued.
“Sam Cross came in last week and I thought he did really well up in Leicester so all of a sudden we are getting more options in the back-row department.
“Guy is still only 27 but has bags of experience at the top end, in the Premiership and European Cup, and with that leadership ability as well, he can do an important job.
“He’ll be available all season round and when you look at those international windows when we lose a lot of experience, having someone like Guy around will be invaluable.
“He’s not coming to just make the numbers up or provide cover, he is a competitor and he wants to be playing in the big games so the increased competition for places will benefit everyone.”
Mercer joins Wales internationals Cory Allen and James Hook as new faces at the Ospreys this season, alongside Cross of Great Britain and Wales Sevens.
He faced the Ospreys with Bath at the Liberty earlier this month in a pre-season friendly and is looking forward to a new challenge over the Severn Bridge.
Mercer said: “It’s a really exciting opportunity. I’m really looking forward to it. It came a little bit out of the blue, but that’s rugby, professional sport, and you have to just roll with it and take these opportunities when they come your way.
“I’ve been at Bath all my career, I grew up supporting them, but this is another challenge for me. I’m excited about what’s in store for me this season with the Ospreys.”
“It’s a little bit strange having played at the Liberty Stadium less than two weeks ago,” Mercer added.
“That gave me a good insight into the squad and what the Ospreys are about, and I’m looking forward to playing regularly at the Liberty this season as part of the home team. It’s a great venue, one of the best stadiums around. It’ll certainly be different for me to be able to call it home.
“I’ve had half an eye on the changes to what is now the PRO14, following what’s been going on over the summer with the South African clubs and that’s pretty cool. It’s a point of difference that the championship has and it’s going to be a good experience for everybody.
“The Champions Cup pool is pretty strong as well, but there’s a couple of clubs I’m familiar with. It’ll be an exciting draw and I’m really excited about the prospect.
“There’s a very strong back-row contingent at the Ospreys with some quality Wales internationals and Lions. There’s going to be a huge challenge just to get a game, but I’m looking forward to it.”
