Thames Water, which is responsible for North Lake at South Hill Park in Birch Hill, is investigating whether the pollution is linked to the theft of diesel from the nearby Sainsbury's petrol station hours earlier.
The company said oil may have gone through drains into the rain sewer system, and from there into the lake.
The theft of diesel from Sainsbury's in Ringmead - up the road from the lake, - took place at 4am on Friday.
That afternoon, bird sanctuary the Raven Haven in Crowthorne and the charity Swan Lifeline were both contacted by walkers concerned about birds collapsed on the lake banks.
Steve Burns, from the Raven Haven, said: "It's a really horrible situation. It's a disaster for the birds."
He added the haven took in two Egyptian geese and three baby coots whose parents died from the oil added: "We are just keeping an eye on the lake. There's quite a lot of active nests down there."
The well-known pair of swans that have been nesting at the lake for many years were taken to Swan Lifeline's centre in Eton after ingesting and inhaling oil.
Wendy Hermon, treatment centre and rescue co-ordinator, said the birds were improving but were having trouble eating and said: "It's really sad."
Bracknell Town Council has been working with the Environment Agency to monitor the situation.
Clerk Mary Harris said: "North Lake is part of the 'balancing pond' system for the area which means that it takes 'run-off' from the roads etc.
"There was a similar pollution incident at Mill Pond last year when a tanker crashed on Mill Lane and part of its load was discharged into the local drain."
There were concerns that oil from the lake could also feed into Mill Pond, and Thames Water put floats along the surface to try to trap the oil.
It also put down 'cushions' to soak up the oil, skimmed the surface into a tanker and aerated the water to help the fish breathe.
Thames Water spokeswoman Amy Weiser added though: "As it would rain, more oil would get pushed into the pond.
"We've been there monitoring the situation."
It has not been confirmed how much diesel was stolen from Sainsbury's but police believe the offenders forced a diesel siphoning point and used a tanker.
Sainsbury's spokeswoman Sarah Dunne said: "The store was closed at the time so no customers or Sainsbury's colleagues were involved. The store reopened before midday."
Call police via the 24-hour inquiry centre on 101 or Crimestoppers anonymously via 0800 555111.