I’m not quite sure what you are asking, but he was found not guilty of murder. To be found guilty the jury would have had to decide the prosecution met its burden of proof “beyond a reasonable doubt and to a moral certainty.” That is a very high standard of proof.
To be found civilly liable of wrongful death the jury needed to find by “a preponderance of evidence”, which is a way of saying by more than half. For punitive damages, which were awarded, they needed to find that he acted with malice by “clear and convincing evidence”, which is more than half, but less than “beyond a reasonable doubt.”
As you can see, the civil case did not require as high a proof, and the plaintiff’s lawyers had the advantage of seeing how the earlier case went, putting OJ under oath before and at trial and had additional evidence in the form of photos of shoes. They also had a West LA jury pool and the issue was not so easily turned into whether the cops were trying to frame OJ.
Why didn’t OJ simply assert double jeopardy? Well because double jeopardy does not apply to civil suits or suits brought by other jurisdictions. Why didn’t he claim the jury verdict exonerated him? You are asking whether “res judicata” or “collateral estoppel” apply. Res judicata means that the matter has already been adjudicated and cannot be adjudicated again. It does not apply because “the People of the State of California” were the party in the first case, not the Goldman or Brown families.
Collateral estoppel is issue preclusion. It can be applied when the parties are not the same when a party (OJ) has had an opportunity to come to court and fully litigate all the issues, and if one or more of the same issues is present in another case, it can preclude it from being relitigated. It requires that the burden of proof be the same or higher for the party with the burden of proof. If, for example, OJ had been found guilty of the criminal matter, it could have been asserted to keep him from contesting the fact that he was responsible for the deaths of the victims.
But in the particular case, he was found not guilty of criminal responsibility. So why weren’t the plaintiff families barred from relitigating? Because they were different parties and the criminal verdict was not binding on them: they were not parties to the criminal case and could not have been.
The fact that collateral estoppel doesn’t apply is because the civil case had a lower burden of proof really never comes into play because of the difference in party identities. But technically the lower burden of proof in the civil case would also allow the families to relitigate.