A fabric's "weight per ounce" actually refers to the fabric's weight in ounces per square yard. For instance, a 6.1-oz. shirt doesn't indicate that the shirt weighs 6.1 oz., but that each square yard of fabric weighs 6.1 oz. Generally, the lighter the weight, the better.
According to the Fabric Blog, a lightweight T-shirt would be somewhere around 5.3 oz., while a standard T is 5.5 oz., and a heavyweight T would fall somewhere between 6 and 7 oz.