You ask an interesting question. When you ask if he is innocent, you pose a question of morality. The jury decided he was "not guilty," but that does not mean Mr. Simpson did not do the deed. It means that, for the jury which heard the case, the government did not prove its case beyond a reasonable doubt.
One person who answered your question said we know he is guilty. While I may believe he committed the crime, I cannot say he is guilty, because the jury found him "not guilty."
Why was he found "not guilty?" It could be one or more of the following:
1) He really is not guilty.
2) The government did a poor job in presenting its case.
3) Government witnesses were not as believable as defense witnesses.
4) Research on juries tells us minorities are less likely to believe law enforcement officials than white. If a police officer and a civilian witness give contradictory testimony, whites tend to give more credibility to the police office while minorities do not give added weight to the police.
5) The defense must raise the question of reasonable doubt which it did.