Written by the parliament's internal auditor, the report analyses 167 payments made in 2004 and 2005 to MEP assistants.
Chris Davies, a member of the UK Liberal Democrat party and who sits on the parliament's budget control Committee, made the request for an investigation after seeing the report.
He was only allowed to view the report under supervision and was forbidden from taking notes or making photocopies.
In one case an MEP, who was not named, was said to have taken money without employing any staff.
"We are now requesting this document from the parliament and are expecting to receive it as soon as possible," an spokesman for Olaf, who wished to remain anonymous, said.
Olaf said that it routinely checks internal audits made by the EU's various institutions, so the fact it had requested the parliament report was unexceptional.
"We could not possibly have any suspicion (yet) because we haven't seen the report," the Olaf spokesman said.
"But what we have heard at least makes the report interesting for us, and that's why we're asking for it, to assess the information in it.
"We have also received requests from MEPs who have seen the report and say it would be interesting for us. So that confirms our decision to ask for it, without knowing what's in it in detail. This is the bottom line: it's worth looking at."
Open Europe is an independent think tank set up by a number British business people to discuss the European Union.
Alistair Tebbit, a research director at the organisation, told Al Jazeera: "We are absolutely astonished at the scale, we are talking about $200m. There are countless opportunities seemingly (for MEPs) to supplement their salary.
"It's just a general problem of a complete lack of transparency and accountancy procedures. The report is being buried and people like you or me cannot see the actual details.
"To compound the problem... Olaf, the EU anti-fraud unit, is itself being investigated by itself."
Despite its increasing sway in drafting key laws, the EU's legislature has often struggled to be taken seriously by the public.
Critics have portrayed it as a "gravy train", with several reports over the years exposing abuse of MEPs' expenses.
European parliament officials said the report confirmed longstanding concerns that the assembly's system of hiring and paying assistants needed to be reviewed.
"This report confirmed that it [the system] has become far too complicated," Marjory van den Broeke, a parliament spokeswoman, said.
There are three methods for MEPs to recruit staff and 27 national taxation, social security and administrative systems in place because some assistants work in Brussels while others work in their MEP's own country.
The European parliament's secretary-general will now propose introducing a single system for administering and recruiting staff, which officials said could come into force after the assembly's next elections in June 2009.