'I was shocked to hear there is to be an internal investigation only. This will be heard behind closed doors and the full findings would not be published.
Did six men fabricate stories?
Intuitively the allegations of the four original complainants and the two additional complaints look plausible, after all there has been one scandal after another involving sexual impropriety by supposedly Celibate Priests  and there have been no scandals involving priests or former priests colluding to make false accusations.
Still what intuitively looks plausible should not lead to a conviction in a court of law.
As the accusations mounted up the possibility that the cardinal wasn't a sexual predator became steadily less credible but as of O'Brien's death in 2018 still nothing had been proved through sworn testimony in court.
One suggestion is as worrying as the accusations.
A source within the Catholic Church also said the cardinal "doesn't know who his accusers are and doesn't know what they're accusing him of". 
(1) Audi alteram partem (Latin for, hear the other side): no accused, or a person directly affected by a decision, shall be condemned unless given full chance to prepare and submit his or her case and rebuttal to the opposing party's arguments;
And high ranking Roman Catholic dignitaries worldwide can understand that because it's also written in Latin. The Vatican considered what to do and  and later made a decision to send O'Brien to an enclosed abbey for an unspecified period of time. The man subject to these proceedings apparently didn't know enough to mount an effective defence. The accused has a right to be heard and needs enough information to prepare an effective defence or rebut charges.  We must take O'Brien's word for things that he really didn't know the charges against him but the cardinal was an experienced member of the Church, his claim that he was denied natural justice is clearly credible and consistent with the way the church does things.
Natural justice applies to courts of law, it doesn't apply to church authorities. Punishments the Roman Catholic authorities can impose onto clergy are as severe as those that courts impose onto convicted criminals, it's frightening that there are no safeguards against abuse.
Why the secrecy?
Why hasn't O'Brien been given details about the charges against him? If he knew more O'Brien could perhaps recognize his accusers, if he's guilty he probably remembers what he did and to which priest. After retiring he can't easily get nasty to his accusers himself but has friends and contacts who could make life very uncomfortable for them. Remember the four who complained can become victims of arbitrary orders or arbitrary justice from superiors as easily as Cardinal O'Brien himself did.
Is there any way of giving O'Brien the information he needs to mount an effective defence and also protecting the accusers? Secular organisations would try and find some way round all this but The Roman Catholic Church apparently thinks it's above ordinary rules like insuring a fair trial.
- Disgraced Cardinal Keith O'Brien facing possible police investigation as it emerges church officials have 'told him to live as a hermit'
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- Cardinal O'Brien's departure leaves a rudderless Church
- Cardinal Keith O'Brien 'very upset' about his resignation
- natural justice
- Pope considering response to alleged 'inappropriate acts' by UK cardinal
- What are the rules of natural justice?