The US is looking for a suitable Mediterranean port where processing can be carried out, OPCW added on Saturday.
OPCW has been under pressure to find an alternative plan for the destruction of Syria's chemical weapons stockpile after Albania backed out of hosting the work.
The OPCW said 35 firms had expressed an interest in bidding for commercial contracts by Friday's deadline for the treatment of about 800 tonnes of bulk industrial chemicals that are safe to destroy in commercial incinerators.
But another 500 tonnes of chemicals, including nerve agents, are seen as too dangerous to import into a country or process commercially, and will be treated offshore on the US ship.
The OPCW said the operation would be carried out on a US vessel at sea using hydrolysis.
The US naval vessel on which neutralisation will take place has not been officially named but is believed to be the MV Cape Ray. It is undergoing modifications to support the operations.
These should be completed by 31 December, the OPCW said.
RA/SHI