Police also targeted sites in Berkshire and Wiltshire this morning.
Warrants were executed earlier to target fraud across the three counties.
Thames Valley Police and partner agencies launched a 'large-scale' investigation following the discovery of financial irregularities after the collapse of a business in Oxfordshire.
Officers haven't named the business, or said where it's based in the county, but four people are now being questioned in connection with a number of fraud and money laundering offences between July 2018 and November 2019.
Two of those were arrested in Oxfordshire, one was taken into custody in Berkshire and another was held in Wiltshire earlier.
Investigating officer, Detective Inspector Duncan Wynn, of the Economic Crime Unit, said: “This morning we have executed these warrants as part of a large and complex fraud investigation.
“Those in custody have been arrested on suspicion of money laundering and fraud offences dating back to 2018.
“As we continue with our investigation, I hope that the action taken today reassures the local communities that we take these offences seriously.”
*WARRANTS*— Thames Valley Police | #StayHomeSaveLives (@ThamesVP) February 26, 2020
Today (26/2) our officers alongside partner agencies have executed eight warrants across Oxfordshire, Berkshire and Wiltshire.
This is part of a large-scale and complex investigation into fraud and money laundering offences.
Details - https://t.co/UYIbkgtunJ pic.twitter.com/sPFpxtWPj0