British media reported on Thursday that the Brazilian and Spanish internationals nearly came to blows after exchanging tackles in a practice session ahead of Sunday's FA Cup third- round tie against Scunthorpe United.
"They were like bulls who were chesting a bit after a charge from behind by Oscar," Hiddink said.
"He got Diego and they stumbled over the ground and then they chested a bit."
Hiddink said other players had stepped between them but that there had been no need for that.
"Costa took a charge from behind from the smaller of the two, Oscar, and it was overdone, especially when you know Oscar is not the biggest tackler of the team.
"They stumbled over the ground, stood up, as a normal reaction. Those things happen when you're close to the edge in an intense session."
Hiddink said Oscar had apologised in front of the other players for over-reacting.
"It was a normal battle," the Dutchman added.
Oscar had tweeted earlier: "He's one of my best friends and we never fight."
Both players endured an indifferent start to the season under former manager Jose Mourinho, but have raised their games since Hiddink took over last month.
They each scored in Chelsea's 3-0 dismantling of Crystal Palace on Sunday, when the champions won for only the sixth time in 20 league games this season.
Chelsea, 14th in the table and 13 points adrift of the Champions League places, host West Bromwich Albion and Everton in the league next week.
(Reporting by Mike Collett in London and Simon Jennings in Bengaluru; editing by Ed Osmond)
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