The Internacional's president Giovanni Luigi has turned down £25m for the midfielder but believes Chelsea boss Roman Abramovich will sanction the extra £5m Luigi wants.
"I want the transfer of Oscar to be the biggest transfer deal ever in Brazilian football," he told Radio Gaucha.
"The Chelsea offer isn't that far from that. Last year, however, Tottenham made two offers for Leandro Damiao. We kept him and he was very important.
"Oscar's situation is similar. If we get offers for Oscar that match our expectations, we'll negotiate. But they have to be of the value that mean significant resources for Internacional.
"But those values would be the biggest negotiation in Internacional's history."