If you can view EPS files in Explorer's preview pane (not in a separate window or as thumbnails, but in the actual preview pane that opens on the side of the normal Explorer windows) then that should also work in Opus.
If it only works in Explorer, please open an Opus window and select Settings -> Preferences / Viewer / Viewer Plugins, then select ActiveX + Preview + Office + Web and click Configure. In the top part of the list there should be a program or component that is assigned to the .EPS extension. What is it?
There is also an EPS plugin for Opus here: Adobe Illustrator, EPS, INDD and PSB thumbs/viewer
Also, the Universal Graphics Viewer Plugin will show the first page of EPS documents if Ghostscript is installed.