The pair are two of five internationals in the 12-man squad alongside Topsy Ojo, Shane Geraghty, and Chris Hala'ufia.
O'Leary replaces Ofisa Treviranus, who was set to be captain, and Tagicakibau steps in for Luke Peters.
Exiles will meet Harlequins, London Wasps and Saracens with the top two teams going through to the finals in Bath on August 6.
Tickets for the event are £10 for adults and £5 for under 16s and are available from www.ticketmaster.co.uk or by calling 0844 847 2492.
Exiles squad: Alex Gray; Chris Hala'ufia; Shane Geraghty; Tom Homer; Topsy Ojo; Michael Worrincy; Gerard Ellis; Darren Allinson; Steven Shingler; Tomás O'Leary; Conor Gaston; Sailosi Tagicakibau.