Both players have impressed in recent weeks for their clubs with O'Brien's form having been rewarded with a call-up to the Ireland Under-20 squad.
Coach Michael Bradley said: "It is a great opportunity to draft John and David in at this level. They both have great potential and it will do them good to come into the side and have a go."
In the back-line Danny Riordan is recalled once again along with two players who have recovered from long term injuries, half-backs Tim Donnelly and Conor O'Loughlin.
Sunday's game sees the eleventh international side visit the Sportsground since touring sides first came west with Argentina in 1973.
With the opening of the new Connacht gym to precede the match on Sunday evening, all supporters have been invited to view the new facility once the game is over.
CONNACHT Squad (v Portugal): Backs (11) - Kieran Campbell, Fionn Carr, Mel Deane, Tim Donnelly, Andy Dunne, John Hearty, Troy Nathan, John O'Brien, Conor O'Loughlin, Danny Riordan, Aidan Wynne.
Forwards (12) - Adrian Flavin, David Gannon, Ronan Loughney, John Lyne, Michael McCarthy, Joe Merrigan, Robbie Morris, Conor Muldoon, David Nolan, Ray Ofisa, Colm Rigney, Brett Wilkinson.