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How can we be sure anyone did anything or was anywhere? Even in cases where there were eye witnesses or DNA evidence, none of that is conclusive in the pure sense, it's just a testimony and occurrence. There's always a possibility that everyone is lying, the video evidence has been tampered with, the fingerprints were planted or perhaps the lab mistakenly identified them as so-and-sos when they were really someone else's. All we can do is determine based on the available knowledge and wisdom of the day how likely it was that a crime was committed based on the evidence.
With the case of OJ, there was a pattern of abusive behavior in his relationship with his ex wife, DNA evidence at the crime scene, and the murder itself based on the condition of the bodies of the deceased and the state of the crime scene suggested that the assailant committed an intensely violent act on Nicole and her friend, not characteristic of a botched robbery etc. Along with OJs testimony and his suspiciously timed travel arrangements, and in the absence of another theoretical perpetrator, there is a pretty compelling case against him.