Jose Mourinho's men are seemingly destined to lift their first title in five years next month, enjoying a seven-point lead over Arsenal having played a game fewer than their London rivals.
But third-placed United visit Stamford Bridge on Saturday on the back of six successive league victories, and Oscar is well aware of the threat posed by Van Gaal's men.
"They were struggling in the beginning but they've shown a lot of improvements," the Brazilian told Perform.
"They are playing very well at the moment. One can see that Man United found their ideal starting XI and we will face a tough opponent this coming weekend."
Although Chelsea look well placed to seal a third league crown under Mourinho, Oscar insists the race is not over yet.
"I wouldn't go that far. It seems easy but it's definitely not," he added. "We will face Man United and then Arsenal. Those will be like two finals for us.
"After that we will have a clear idea on how close we are from the title. We will certainly fight until the end of the season. But we hope we can be the champions as soon as possible."
The form of Marouane Fellaini and ex-Chelsea man Juan Mata have been two bright sparks in United's recent run, but Oscar says their rivals have dangerous players all over the pitch.
"It's not only Fellaini but Mata as well. [Ander] Herrera, [Daley] Blind, [Wayne] Rooney, Ashley Young, so many players," said the 23-year-old.
"They certainly improved their performances at this point of the season and we must be careful with all of them. But they must be careful with us as well so big derbies like this one are always great matches."
