You can reproduce the problem by starting a render, and then while rendering resizing the viewport that is rendered.
Unless I am m misunderstanding your problem - this is the intended functionality, and the "roaming Rhino camera" is a Rhino issue rather than an Octane plugin problem (
http://v5.rhino3d.com/forum/topics/view-lock and
https://discourse.mcneel.com/t/how-can-lock-perspective-view-copy-a-text-from-rhino-and-edit-this-text/4813/6). The plugin sync's the Octane camera to the Rhino camera position of whichever viewport you have selected in Rhino. The Rhino camera position is panning when you change the Rhino viewport width. As Dottedline indicates, you should do the Octane render from a new viewport which always has the same aspect ratio - which will get around this issue....
1) Create New Viewport
2) Set View->Perspective
3) Save the new view as a named view. If you click Lock View, each time you select this view it will revert to the same camera position (
http://docs.mcneel.com/rhino/5/help/en-us/commands/namedview.htm)
4) Open the Viewport - which will use the new named view if selected
5) Enter the second two commands listed by DottedLine above if you want the named viewport fixed to a certain size
6) You can click the Lock View button on the Octane Viewport to stop the camera changing to other Rhino viewports when you click on them
Paul