Ireland flanker O'Brien has scored four tries in the Heineken Cup this season and will line-up against Toulouse at the Aviva Stadium next weekend.
Leinster's Nacewa has excelled at full-back this season as Rob Kearney has been absent through injury since November. The Fijian has scored 28 points, including 3 tires in the Heineken Cup during this campaign.
Former Heineken Cup winner Keith Earls has played in all seven of Munster's European games this season and has scored four tries, including a brace in the quarter-final victory over Brive.
Meanwhile, Munster scrum-half Conor Murray, Leinster back-row Rhys Ruddock and Ulster centre Nevin Spence are the candidates for the IRUPA Young Player of the Year.
The winners will be announced at the players' body awards ceremony on Wednesday, 4 May