A pot and 5,000 historic bronze coins dating back 1,700 years unearthed from farmland at Chalgrove have been officially declared treasure.
Metal-detecting enthusiasts Brian Malin, 30, and his sister Carol, 37, came across the hoard last April.
One of the coins discovered is extremely rare as it is only the second of its kind ever found, and confirms the existence of the rebel Roman emperor Domitianus or Domitian II.
The find made national headlines earlier this year, and the coin, which bears the face of the emperor, has already been on display at the British Museum.
At a hearing at Oxford's Coroner's Court, the 4,957 coins - which date from 251AD to 279AD - and the pot they were found in, were officially declared treasure by Nicholas Gardiner under the 1996 Treasure Act. This followed an assessment of the items by British Museum experts.
The coins are to be valued by the Treasury Valuation Committee, as by law Mr Malin is entitled to their true market value - a figure expected to be several thousand pounds. Mr Malin has an agreement with the landowner Chris Nixey that he will give him half the amount he receives.
It is then hoped the pot and coins will be returned to Oxfordshire to be exhibited.
Mr Malin, of Cotswold Crescent, Chipping Norton, made the find on April 4 last year. About 10 years ago, he came across a hoard of Roman coins in a similar location scattered from two broken pots. But this time the pot containing the coins was intact, buried about 18 inches below the surface.
Mr Malin, a supervisor at plastic injection moulding services company OGM at Eynsham, said he got a faint signal, which got stronger as he dug down deeper.
Miss Malin, of Caulcott, near Middleton Stoney, said they both assumed the item would turn out to be a plough shear which had fallen off and become buried.
But to their surprise, they saw the rim of a pot full of green-coloured discs, which Mr Malin carefully lifted out.
"I was completely shocked," he said. "I wasn't expecting to find another hoard."
Volker Heuchert, collections manager at the Heberden Coin Room, at the Ashmolean Museum, Oxford, said: "It is only the second known example of a coin by Emperor Domitian II. It confirms the existence of this emperor. Until then only a single coin had been known of, and there are always doubts about single coins."
Emperor Domitian II ruled the Gallo-Roman empire, which included western Germany, France, Spain and England, around 270AD.
Nick Mayhew, deputy director of the Ashmolean, added: "Their behaviour was exemplary in reporting the find promptly. This is an ideal example of how the Treasure Act should work."
He said that the museum was very keen to eventually secure the hoard to exhibit in Oxford and at the county museum in Woodstock.
