Ireland take on Turkey at the Aviva Stadium, Dublin, on May 25 followed six days later by a showdown against Italy at Fulham's Craven Cottage.
Costa Rica are next up in the in PPL Stadium, Philadelphia on June 6 before O'Neill brings the curtain down on Ireland's season with a game against Portugal at the Metlife Stadium, New Jersey on June 10.
The squad will be shortened ahead of the squad meeting up on May 22.
"These four games will provide us with great preparation for our first European qualifier against Georgia in September," said O'Neill.
"I have enjoyed working with the players to date, and intend to use this extended period with them in May and June to give me the opportunity to experiment and familiarise myself with the squad further."
Squad: Forde (Millwall), Randolph (Birmingham), Elliot (Newcastle), Keogh (Derby), Wilson (Stoke), Coleman (Everton), O'Shea (Sunderland), Pearce, Stephen Kelly (both Reading), Delaney (Crystal Palace), O'Brien (West Ham), Duffy (Everton), Clark (Aston Villa), Ward (Brighton); McCarthy (Everton), Hendrick (Derby), Whelan (Stoke), Green (Leeds), McGeady (Everton), Pilkington (Norwich), McClean (Wigan), Quinn, Meyler (both Hull), Hoolahan (Norwich), Keane (LA Galaxy), Long (Hull), Murphy (Ipswich), Walters (Stoke), Doyle (QPR), Stokes (Celtic), Cox (Nottingham Forest), Sammon (Derby).