The two sides face each other at Wembley on Sunday in an intriguing League One play-off final, with an estimated £5million the financial reward at stake for the team who return to the second tier of English football.
Should Rotherham clinch the last remaining place in the Championship for 2014-15, it would continue a remarkable rise back up the football pyramid under manager Steve Evans.
The Yorkshire club were 10th in League Two when Evans took charge in April 2012. Automatic promotion followed in the Scot's first full season in charge and he very nearly repeated the feat this term, but ultimately fell just short.
Nevertheless, a 4-2 aggregate semi-final win over Preston North End means Evans is one game away from following in the footsteps of legendary former Rotherham boss Ronnie Moore, who masterminded successive promotions from the fourth tier to the second.
Given that both Rotherham and Orient finished level on 86 points this season and won their respective home games against each other, picking a clear favourite is tricky, as Evans himself is quick to concede.
He said: "I have said Leyton Orient will be favourites, Russell Slade has said we are favourites. Many neutrals would say there is not a lot in it.
"They led the table for seven or eight months in the season, many people thought they wouldn't be caught.
"They finished the season in terrific form so I think they are the favourites but I would accept that this is the one play-off final that could go either way."
Evans may be quick to shower Orient with praise, but his Rotherham side have suffered just one defeat in 24 games.
Orient, meanwhile, are bidding to end a 32-year exile from the second tier and can count themselves unfortunate not to have sealed a top-two finish, having amassed 86 points, more than League One champions Doncaster Rovers claimed the previous season.
Slade – who lost the 2006 League Two play-off final during his spell in charge of Grimsby Town – said: "We feel very proud of what we have achieved this season, 86 points is a fantastic effort.
"Five out of seven seasons that would have got you automatic promotion."
Slade has been linked with the vacant managerial position at West Brom in recent weeks and victory at Wembley on Sunday will likely fuel speculation of a potential move to The Hawthorns.
Orient reached the play-off final after beating Peterborough United 3-2 over two legs.
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