London Mayor Boris Johnson commissioned the three-week review.
"It is clear also that on costs and funding we need to have greater transparency and openness," he said.
"The ODA's final cost estimates already anticipate that at least a billion pounds of contingency provided within the Budget will be spent.
"I want to make it clear that Londoners will not pay a penny more in council tax to pay for the games no matter what the circumstances," he added.
Mr Ross, who sits on the London Olympic organising committee as Mr Johnson's nominee, was asked by the mayor to keep an eye on the Olympic purse strings.
He identified questions over the security of the 2012 Games, its legacy and the Olympic Village as key areas which need to be "focused on".
The report points to tighter controls on bank lending, falling house prices, increased oil and raw material costs and the threat of terrorism as factors which will add "significant pressure" to the cost of the Olympics.
Boris Johnson praised the progress made by his predecessor Ken Livingstone
Mr Ross noted difficulties in securing finance for the Olympic Village and said work was ongoing with the developer, Lend Lease, and its banks to secure private equity and debt funding.
He warned: "In all scenarios some significant additional public sector funding is likely to be required to deliver the project."
However, he said savings made elsewhere meant there "has only been a very small increase in the overall anticipated final cost of the total programme of £16m."
He said security plans were "significantly behind the rest of the planning".
"It is absolutely vital that significant progress is made quickly on security planning so that necessary facilities are identified early enough to be provided cost effectively," he said.
And while he said there would be transport and regeneration benefits from London hosting the games, he issued a word of caution with regards to a lasting legacy.
"I am disappointed that we find ourselves at an advanced stage of the procurement process without clear and agreed plans for legacy for a number of important venues," he said.
The ODA spokesman said: "The net cost pressure on the overall budget is £16m so it is not correct to talk about a £100m increase in costs as has been reported in some areas.
"75p of every pound we spend is for long-term regeneration so a great deal of work is already being done to create a lasting legacy."
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