The OOps window (I don't remember what it stands for) shows the relationship of objects in Blender. I don't know when it happened, but nowadays it's a view option for the Outliner.

Switch a window to the outliner, and if you look under the View menu, there is an option to let you switch between outline view and the Oops schematic.

Blender has been changing a lot since it became open source, and a lot of the tutorials out there are for early versions of Blender 2.0, so don't be surprised if a lot is different. (Personally, I think they should have changed at least one major number rather than just changing the minor number, considering the massive changes - sometimes I feel like almost nothing in Blender works quite the same nowadays as it did back when NaN still owned the thing a couple of years ago, and they still haven't gone 3.0)