In the case of Hill v. Skinner (81 Ohio App. 375, 79 N.E.2d 7887 (1947)), Hill was a 4 year-old boy who claimed that he wandered into Skinner's yard and was bitten by Skinner's dog.  While Hill had some cuts and bruises, there was no proof that he suffered from a dog bite, and there was no other evidence outside of Hill's testimony to prove he had been bitten. Based on Hill's testimony alone, Skinner was found guilty. The Trial Judge felt that Hill seemed to know the difference between the truth and a lie, and the ramifications of lying.  He appeared to be able to relate events and was deemed competent. The Appellate Court affirmed, saying that under Ohio State common law, it is within the judge's discretion to decide whether a small child is a competent witness or not. "The essential test of the competency of an infant witness is his comprehension of the obligation to tell the truth and his intellectual capacity of observation, recollection, and communication." This case was decided under the common law.  Today it would be governed by FRE 601.