Baron Oublenmach is first seen in joint command of the very pink airship of Zola Heterodyne as it floats over Mechanicsburg. Greatly confident that their planned manipulation of the current turmoil will allow them to take control of Castle Heterodyne and the town, he argues with his partner Strinbeck that they are prudent to seize the opportunity presented to them. It is very likely that he and his co-conspirator are Knights of Jove, especially from to Gilgamesh Wulfenbach's use of lightning in battle.
Unlike Strinbeck, he is (evidently) not present on the Airship when it is attacked by the Torchmen and then confronted by Castle Wulfenbach. (Of course, Strinbeck is also no longer on board by the end of this sequence..)
He resurfaces in the narrative having broken into the house of Vanamonde Heliotrope, holding the latter man's family and self-appointed secretary hostage and demanding that Vanamonde ring the Doom Bell. He reveals that he is motivated solely by a chance to loot the fabled Heterodyne treasury, a plan that requires the Doom Bell to be rung, and he actually had no ideological investment in the plot to install a false Heterodyne.
Learning that the Bell has been disabled because of earlier treasure hunters via the removal of its massive striking hammer, he demands to be told the latter object's hiding place, which turns out to be a certain famous bar. After receiving a lesson in manners, he politely requests that the bar's owner give the hammer to him, which she does. He then literally drags it across town to the Doom Bell tower, where it is successfully deployed by one of the Bell's mechanical attendants.
When last seen, he is laying prostrate less than 10 feet from the newly rung Bell, having had showered upon him a clawful of gold from treasure-guardian Franz Scortchmaw in return for his efforts. As seen in the included picture, he considers this to be "winning". Presumably he is trapped inside Mechanicsburg when Klaus deploys his Take-Five Bomb.
Oublenmach is an overweight, shortish older man, whose vest appears to be metal with gauges set in it, and his hands are always shown either covered with metallic gloves or replaced entirely with mechanisms. Given the conventions of the royal families of Europa, it is unlikely his is some kind of cyborg or construct. (Although see below.)
Agatha H. and the Voice of the Castle offers a couple more details about the man: he is a common-born long-time schemer and soldier of fortune who bought his current title with some of his ill-gotten gains. Also, he is explicitly shown leaving the airship for the surface via drop-line before the vessel is forced to flee the Torchmen and encounters Castle Wulfenbach.