There are two possible verdicts, Guilty or Not Guilty. If the verdict is Guilty the defendant would be pronounced guilty as charged. Therefore, if the verdict is Not Guilty the defendant must be innocent of the charges against him/her by default. No other interpretation is possible. .
The person can be innocent of the charges but not innocent of the act. One can conclude that OJ was innocent of the charge of murder, however that does not mean that he didn't murder. The court decided that OJ was not convicted of murder, technically meaning he was not found guilty. But he did still murder two people.
Just as it's possible that a person in jail committed no crime (because they were falsely declared guilty) it is possible, and in fact much more likely, that someone declared not guilty is indeed guilty. In the eyes of the law though OJ was "not guilty", he was not "innocent".
"Innocent until proven guilty" is the foundation for the legal system however if I recall the language used is more well bounded. I forget the exact wording but it's something like "presumed innocent until proven guilty in the eyes of the law". This means that the law can not punish until guilt is proven. Notice it does not place limits on individuals: I may conclude OJ is guilty even if his guilt isn't proven. The slander example is a good one: I may say OJ is guilty without risk of slandering him even though his guilt was not proven. The civil case is also a good one: OJ could not use his lack of conviction as his only evidence against the civil case and subsequently lost, despite not being convicted.