I think the Hokage being 'the strongest ninja of the village' is meant to be a measurement of the total package (Intellect, Patience, Honor/Heart, Fighting Skill), not just simple physical combat ability. Similar to how almost any political leader (even ex-military) isn't expected to be the toughest fighter around.

Think of it in terms of the Sand Village .. Gaara/Shukaku is almost unquestionably the most powerful fighting force there, but are in NO way capable of performing the duties of Kazekage, due to being .. oh, what's that word? Oh yeah .. INSANE [img]i/expressions/face-icon-small-smile.gif[/img] This ENTIRELY contadicts the notion that the Kages are necessarily the strongest fighter.

Similarly, seeing as how Oro almost got owned by the 3rd, even when he had him outnumbered with TWO other hokages, and the 3rd really wasn't even impressive at all in that battle (no taijutsu really, no massive summoning, nothing that really represented much of a threat other than the sealing) .. Oro looks really weak.

This leads me to believe that Oro's strengths are his intellect and knowledge of jutsu, he just doesn't seem to be much of a physical fighter. The 3rd, 4th, Jiraiya, and probably Itachi are all head and shoulders above Oro in terms of fighting, but Oro is trickier, and uses his wide knowledge and intellect to overcome most of his weaknesses where he can find ways around his weak points.

As I said, there is ample evidence that Hokages are NOT necessarily the most powerful fighter, but rather the best choice for leader of their nation