Jim Tressel officially turned in his letter of resignation to Ohio State athletic director Gene Smith on Monday, marking an end of an era and opening the door to many possible replacements.

OSU announced in a press release that linebackers coach and co-defensive coordinator Luke Fickell will be the interim head coach for the 2011 season. The statement said a search for a new head coach has not begun.

“Recruitment for a new head coach — which is expected to include external and internal candidates — will not commence until the conclusion of the 2011–2012 season,” President E. Gordon Gee said in the press release.

While the university does not officially plan on searching for a replacement until the end of the season, the program could have several coaches on its radar already.

Urban Meyer, former Florida coach

When the news broke Monday morning that Tressel had submitted his letter of resignation, the Twitterverse exploded with speculation. Immediately, OSU fans, college football fans, analysts and speculators began tweeting that Meyer would, and should, be Tressel’s successor. It came as a surprise to none that “Jim Tressel” was trending in the U.S. What people might not have expected, however, was for “Urban Meyer” to trend nationwide for most of the day as well.

Tressel and Meyer have contrasting styles of offense: Tressel’s patented “keeping it close to the vest,” and Meyer’s “high-powered” spread offense. The success of both is undeniable.

In his relatively short career, Meyer boasts a strong résumé that includes two three-loss seasons at Bowling Green, an undefeated season at Utah and two national championships at Florida — one at the expense of Tressel’s 2007 Buckeye squad.

Meyer released a statement Monday regarding the news.

“I am committed to ESPN and will not pursue any coaching opportunities this fall,” Meyer said. “Jim Tressel has been a respected friend and colleague for a long time. I wish Jim and his family the very best now and in the future.”

Luke Fickell, OSU interim head coach

Fickell will be the interim head coach for the 2011 season. However, the role Fickell will play after next season is unclear.

Fickell knows the system as well as anybody. He has been in Tressel’s system for nine years, serving as special-teams coordinator, defensive line coach, linebackers coach and co-defensive coordinator.

He also played for the Buckeyes from 1992–96 as a nose guard, finishing his career with 212 tackles and six sacks. He was on the 1996 team that won the Rose Bowl against Arizona State.

Fickell has no prior head-coaching experience, so he has one season to show OSU what he is capable of.

Mark Dantonio, Michigan State coach

Dantonio left the Buckeyes in 2004 to pursue a head-coaching position at Cincinnati. He was defensive coordinator at OSU from 2001–03, including the 2002 National Championship against a heavily favored Miami (Fla.) squad.

Dantonio and his defensive staff were able to shut down the Hurricanes’ offense, which included current NFL players Andre Johnson, Kellen Winslow and Willis McGahee and notable college quarterback Ken Dorsey.

Because of his previous experience with the Scarlet and Gray, Dantonio is familiar with the Buckeyes’ system, and the transition would be easy for the players.

Dantonio now coaches at Michigan State and led the Spartans to an 11-2 record last season, claiming a share of the Big Ten Championship.

Jon Gruden, former NFL coach and ESPN analyst

Former Super Bowl champion Gruden has proven over the past few years that he has the charm and savvy to talk football on TV. But this coach could be the fit OSU is looking for in its next coach.

Gruden already has two years of experience helping NFL-bound quarterbacks prepare for their future. Gruden’s ESPN miniseries “QB Camp” exposed Gruden on a national level as a charismatic character, a quality that can propel him to the job as a vicious recruiter.

Gruden visited OSU in late April and spent a few days watching and studying the Buckeyes. He spoke about freshman quarterback Braxton Miller, the perceived front-runner to start in 2012.

“They’ve got a hotshot guy in here from Dayton. No. 5 is all I know,” Gruden said. “Braxton is his name. He’s pretty darn good too.”

Bo Pelini, Nebraska coach

Pelini was raised in Youngstown, Ohio, and attended OSU, playing free safety under coaches Earle Bruce and John Cooper. Pelini was co-captain of the Buckeyes his senior season and graduated from the university with a business degree in marketing.

Pelini spent eight years as an assistant coach on three NFL teams, and since has become the coach at Nebraska.

Pelini’s ties to OSU give the Buckeyes the option to pursue him, but a recent five-year deal with Nebraska made him the second-highest-paid coach in the Big Ten.

Adam Hawkins contributed to this story.