The following kashrus information is from the OU in response to the editor's question on July 9, 2015.
Many Polly-O Mozzarella Cheeses are now certified by the OU when bearing the OU-D symbol in addition to Polly-O and Kraft String Cheeses. Look for the OU-D.
The previous item can be cited with the URL: https://www.kashrut.com/NewCerts/?alert=N0200