Use keyword searches as per the Search tab in this guide.
Try using a composer's name work "work" or one of the other language variants such as "werk', "gesam", "sammlung", "ausgab" or "oeuvre". Similarly, if looking for an edition of a specific musical form such as motet, foreign variants may be applied or use truncation (in this instance): "motet*" which will also yield results for "motetta"
Many composers have series of Collected Editions dedicated to their works. These are located at CE 780.81 followed by the first four letters of the composer's name. This can be easier than using catalogue searches in some cases.
Some composer entries in Oxford Music Online include details regarding the Collected Edition for specific works. An explanation of the editions and series is included at the beginning of the works list.