A total of 539 people are being taken off the Safe Scandinavia accommodation platform in an operation involving 13 helicopters and a fixed wing aircraft.
It is understood a bomb disposal team has been sent to the platform, although it has not been confirmed that a device has been found.
The operation, 130 miles off the Aberdeen coast, began at 0920 GMT.
A spokesman for Aberdeen Coastguard said it was a "large scale" alert and that two platforms were involved, but that just one was being evacuated.
The workers are being transported to rigs in the area.
'Unusual incident'
Britannia Operator Limited, which owns the platform, confirmed that reports were being investigated of an incident involving the Flotel Safe Scandinavia.
It said the relevant emergency authorities had been alerted and a company response team was assisting in the co-ordination of support services.
The company added that it was doing everything possible to resolve the situation.
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The Ministry of Defence said five RAF helicopters were involved in the operation, along with coastguard aircraft.
Squadron leader Barry Neilson, from RAF Kinloss, said of the incident: "It's unusual, that's probably the best way to describe it.
"But it falls within our normal operating procedures and we are reacting accordingly."
Mr Neilson said that there was a report of a woman on the platform who had sparked the incident, but he could not go into details at this stage.
The Press Association reported that a "device" had been found on board.
However, Mr Neilson said "we are not sure about a device".
