I think you mean millilitres not millimetres. I also think you mean US measurements of fluid ounces rather than imperial measures. There are 28.4 ml to a fluid oz (imperial measure) or 29.6 ml to a fluid oz (US).
Millimetre or millimeter is a measure of length; ounce is a measure of weight; fluid ounce is usually associated with volume, as is millilitre or milliliter. So we have to compare like with like, thus we compare volume in one unit to volume in another unit. One ounce is taken to be the weight of one fluid ounce of certain liquids, usually wine or water.