Bookmarks are a great way to save an specific page of the oBook for later reference. Octavo also allows you to include notes to each bookmark.
Once you have opened an oBook, you can access its bookmarks from the top right dropdown. This is how it looks like.
From the reading area, scroll from the top of the screen to reveal the top bar. Find the bookmark icon and then include your notes, if so, on the dialog that appears. Tap outside or enter to save.