Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "ofer-slype." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 661.
, es; m. An over-garment, surplice :-- Ðæt mæssepreósta ǽnig ne cume binnan circan dyre búton his oferslipe (-slope), L. Edg. C. 46; Th. ii. 254, 10. He is ymbscrýd mid hwítum oferslype he is clad in a white upper garment, Homl. Th. i. 456, 19.