Technically , taxonomies could be assigned by comparing 16S data ( I.e. OTU in your case ) to publicly available databases like green genes or SILVA. However, with a small stretch of gene like 16s, it will be unfair to declare taxonomies up to species level. The reason for that is, there are many organisms which do share high degree of similarity for this gene , but taxonomically they are very much different at species level. There are also cases where 16s regions are quite different but the spices or genus is same. Hence, a classification upto family level is acceptable , and further assignment down the taxonomic domains is a matter of careful consideration, analysis of other marker genes and wet lab validations. By the way, OTU itself is a highly mis understood term, with vague and often mis used meanings.
