Ohio State has a shortened season which puts complications on if they would even make the playoffs with one loss.
Ohio State is a heavy favorite to win each game, win the Big Ten Championship, and return to the College Football Playoffs. Nobody who has kept up with this team would be surprised if Justin Fields won the Heisman and the team ended up as National Champions. But, season rarely go as planned and we still have to play the games. What happens if Ohio State loses a game?
The two most likely games Ohio State could lose are against Penn State and Michigan. In a shortened season this would give the tiebreaker to the Nittany Lions or Wolverines and the Buckeyes would need that team to lose twice to make it to the Big Ten Championship game. Without non-conference opponents to boost a resume, a team that did not win their conference should have no chance of making the playoffs, especially a Big Ten team that is 7-1.
So that means Ohio State would need to lose to not Penn State or Michigan to still play in the Big Ten Championship game and have a chance. Unfortunately, every other game would be a bad loss. The Buckeyes do not have other teams that are likely to be ranked by the end of the season on the schedule. If it is a close loss, maybe the committee would look at an 8-1 Ohio State team, but chaos would have to happen elsewhere.
Clemson would need to lose the ACC or lose twice during the season (not likely), the Big 12 would need to have a two-loss champion (more likely), and the SEC would need to have one team dominate the conference and have every other team end the season would two losses (possible). Unless a conference has two teams meet in the conference title game without any losses it seems likely that the four conference winners make the playoffs – as long as they have only one loss.
Ohio State can easily avoid this scenario by going undefeated, but as long as the Buckeyes beat the Nittany Lions and Wolverines they can probably afford one loss, assuming the SEC (aka Alabama) has one team that wins handily. If the SEC has two undefeated in their conference championship game they likely both get in and Ohio State would not make the playoffs.