oru is a Japanese existence verb roughly corresponding to the English verb "to be." Like the very similar Japanese existence verb iru, oru can only be applied to animate beings, such as humans and animals.
In Modern Japanese, the verb oru is perhaps best known in its polite form orimasu, which serves as the humble form of the much more commonly used iru in Japanese's complex system of humble and honorific speech (keigo). However, in certain western Japanese dialects, such as the Kansai dialect, oru is used in place of iru in certain situations to convey a sense of disdain for the subject. Thus in these dialects, rats, insects, and stray dogs are often described using oru, and using oru to describe a person would constitute a grave insult.
Interestingly, the modern verbs iru and oru both descended from the same Old Japanese verb wiru. In Old Japanese, wiru was an extremely common verb meaning "to cease movement." As with iru and oru, wiru could only be used to describe animate objects. However, unlike in modern Japanese, wherein "animate objects" only includes humans and animals, in Old Japanese the verb wiru could also be used for forces of nature such as wind, waves, clouds, etc., which definitely says something interesting about where the ancient Japanese drew the line between animate and inanimate.
In Old Japanese, the one and only stative form of the verb wiru was wori, which was a contraction of wi-ari ("has ceased moving"). However, over the centuries, a second stative form of wiru began to appear. This was the form witari, which was a contraction of wi-te-ari ("is ceasing to move"). Over time, these stative forms increasingly came to take on the sense of "to be," as a sentence which literally meant something like "He is over there not moving" increasingly came to mean simply "He is over there."
For a time, both of these forms coexisted, but after a while, witari replaced wori in eastern Japan, whereas it either died out or never even developed in much of western Japan. witari was eventually shortened to wita, then lost its /w/ to become ita, and was finally regularized as a new existence verb iru. Similarly, in the west, wori lost its /w/ sound to become ori, and was eventually regularized as the existence verb oru.
The final chapter in the evolution of the modern Japanese verb oru occurred in the Edo Period, when Tokugawa Ieyasu established his capital in eastern Japan in the fishing village of Edo (modern-day Tokyo). As the administrative capital of Japan, Edo soon grew into a large metropolis, but many of its denizens were warriors from western Japan, where oru was the most common existence verb, whereas the locals more commonly used iru. Accordingly, oru came to be associated with the samurai class and took on dignified or arrogant connotations and thus when local people wanted to show respect, they started using the verb oru in imitation of the samurai, and hence we have the modern humble form "orimasu".
Finally, as Tokyo Japanese became the standard language of Japan in the 1800s, and its primary existence verb, iru, made its way to western Japan, iru came to be seen as the "proper" verb for "to be," whereas the local verb oru came to be seen as crass and vulgar, hence the modern sense of using it only to insult or deprecate.