Oxford Music Online, also known as Grove Music Online, features "signed" articles by experts in their fields. The citation format provided here is from Chicago 14.132, which allows such encyclopedia entries to be treated like contributions to a multiauthored book. You do not need to provide the specific URL/link for your entry since it will not be accessible to readers outside of IU. Just use the general http://www.oxfordmusiconline.com. Deane Root is the editor for Oxford Music Online.
F: Edward Strickland, "Glass, Philip," in Grove Music Online, ed. Deane Root (Oxford University Press, 2013), http://www.oxfordmusiconline.com.B: Strickland, Edward. "Glass, Philip." In Grove Music Online, edited by Deane Root. Oxford University Press, 2013. http://www.oxfordmusiconline.com.
If you are consulting an entry from an in-print edition of Grove, you will need to include the volume information (usually in Roman numeral). You will also need to cite the editor(s) of the edition you use. In the footnote, include the page number(s).
F: Edward Strickland, "Glass, Philip," in The New Grove Dictionary of Music and Musicians, eds. Stanley Sadie and John Tyrrell (Macmillan, 2001), ix: 934.