O’Neill, who skippered the Rebels in 2011, and Cronin, the county’s Munster SHC winning captain in 2014, have been deemed surplus to requirements by manager Kieran Kingston.
The Cork supremo has trimmed his squad following last Sunday’s Allianz Hurling League Division 1A relegation playoff victory over Galway, with defender O'Neill let go along with four forwards.
Kingston's cut will be viewed in some quarters as a statement of intent as he prepares to invest heavily in youth.
It is understood, however, that O’Sullivan informed Kingston last week that he wishes to take some time out from intercounty hurling, while McCarthy is believed to have opted out himself due to work commitments, having been unable to attend all of this year’s training sessions.
The five players racked over 100 championship appearances between them, with O’Neill (30) and 28-year-old Cronin each featuring 35 times for the Leesiders.
Cronin, O’Neill and O’Sullivan all made their senior debuts for Cork in 2007, McCarthy featured for the first time in 2009 and Moylan made his bow in the 2012 All-Ireland SHC qualifier against Offaly.
McCarthy also opted out of the Cork squad last year, with then manager Jimmy Barry-Murphy explaining that the player wanted to take a break “due to work and other commitments.”
McCarthy, 26, returned to the squad this year under Kingston but his League time amounted to just ten minutes as a substitute in the opening round victory defeat to Galway, when he scored a point after coming off the bench.
None of the five players who have departed the squad featured against Galway last Sunday but O’Neill started against the Tribesmen earlier in the season, and also against Waterford and Tipperary.
O’Sullivan, meanwhile, also started the opening fixture with Galway but was restricted to substitute appearances for the remainder of the campaign in the defeats to Waterford, Dublin, Kilkenny and Tipperary.
O’Sullivan, 27, scored 1-2 in this year’s League campaign, two points against Galway in February and a goal off the bench in the Tipp game just over a fortnight ago.
Moylan, 28, had three substitute appearances in the League, against Waterford, Kilkenny and Tipp, but didn’t start any of Cork’s six fixtures.