The French National Rugby League fined Williams, who played for Paris club Racing 92, for "behaviour likely to undermine the image and the reputation of rugby", the governing body's disciplinary committee said in a statement.
He was also ordered to undertake a behavioural training course.
The 35-year-old was fined 1,500 euros by a local court earlier this month on charges of purchasing cocaine.
O'Connor, who was arrested with Williams by Paris police near a nightclub during a night out, escaped a fine but will also need to do the training.
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Williams was suspended by Racing 92 after his arrest and the club later said he would be fired.
O'Connor was also suspended by his club Toulon but has since resumed playing for them.
(Reporting by Ian Ransom in Melbourne; Editing by Amlan Chakraborty)
