Oscar is Popeye's balding, buck-toothed, none-too-bright friend that holds a number of odd jobs throughout his appearances, and sometimes accompanies the sailor protagonist on his adventures. He appeared in the original strip Thimble Theatre by E. C. Segar, and by Tom Sims and Bela Zaboly.
Oscar is something of a gag character in Popeye media, often appearing as a "Where's Waldo?" of sorts in backgrounds or in prominent scenes with a different job and (usually) outfit each time he appears.
Creation and development in Thimble Theatre
Oscar was created by newspaper cartoonist Elzie Crisler Segar, as a minor recurring character in the strip.
After his appearance in Thimble Theatre, Oscar would return as a minor supporting character in the 1948 comic book series by Segar's assistant Bud Sagendorf. His comic book appearances would continue for decades until the title's end in 1984.
As Popeye's popularity greatly grew, he would be given his own animated adaptation by Fleischer Studios. Oscar himself would also go on to become a minor recurring character in these shorts where he made semi-regular appearances with a variety of minor supporting roles where he had a different occupation each time, such as The Spinach Overture and Let's Celebrake, among others.
The All-New Popeye Hour
In 1978, Hanna-Barbera Productions with King Features Syndicate would produce a new Popeye television series: The All-New Popeye Hour. Oscar remained unchanged and would make semi-regular appearances and cameos in several episodes with a different occupation each time, much like in the Fleischer-era shorts.
Oscar would go on to have a minor but notable role in the show's Valentine's Day special, The Popeye Valentine Special: Sweethearts at Sea.
Popeye's first movie
Oscar would return in IDW Publishing's 2012 Popeye comics revival as a semi-recurring character, once again having a number of different odd jobs in every issue he appears in, ranging from such things as a simple shop clerk to a royal chauffeur.
Oscar's gallery can be viewed here