The communication from Trinamool Parliamentary Party leader Mukul Roy comes ahead of the monsoon session of Parliament beginning on August 8.
The role of the chief whip is to ensure that members of the party attend and vote as per the decision of the party.
The Rajya Sabha secretariat has been informed that O'Brien would attend, on behalf of the party, the meetings of House Business Advisory Committee besides other business of the House in the absence of Roy. O'Brien, a popular TV personality who hosts quiz shows, joined Trinamool Congress in 2004.
Currently, the vice president of the party, he has been a member of Rajya Sabha since August last year. The party has nine members in the Upper House of Parliament.