I think it's somewhat difficult to answer your question with a simple answer without knowing why you're asking. Depending on your reason, the answer can vary dramatically.
If you're asking because you want to enter a model in a contest, the answer is that Out-of-the-Box (OOTB, or OTB for short) rules are established by each event. While they may be the same or extremely similar to other events, they do not have to be the same and many local organizations have their own rules on what is or is not OTB.
Within IPMS/USA, there has been a great deal of discussion regarding the OTB rules (and from the rumors, a major change is coming soon) primarily because you can now buy an armor kit - for example - with photo-etch parts, a metal barrel, and even resin components included in the box. Under the old "spirit of the rule", none of these items would be authorized and still have the model entered as an OTB model. However, because the rule was not substantially changed, these kits have been eligible for OTB awards at the last two IPMS/USA Nationals. As I said above, look for changes in the IPMS/USA OTB rules soon.
If you're asking your question just to know what modelers' mean when they say, "OTB", that's an even harder question to answer. Each modeler has their own personal idea of what does and does not mean OTB. Some modelers think adding monofiliment antenna or rigging to aircraft (as long as it's shown on the box art) is perfectly acceptable while others do not. Most consider tape seatbelts in cars and aircraft as allowed under OTB rules...a few do not. The number of differences like these could fill several pages.
In general, as stated above, the "spirit" of the meaning for OTB is that nothing other than the parts which came with the kit are used and only your own modeling skills to thin parts to scale, clean up all manufacturing defects, properly assemble the kit, and finish the model are used.
I hope this helps...