An Oil Drum is a type of liquid storage container similar to a Barrel. It is often used at times as an improvised Bathtub in poor country side environments. This is a very informal version of the Goemonburo. Usually water is poured into it and the drum is held up over a fire where the water can be heated.
Son Goku, Son Goten and Trunks have a bath in the oil drum at Goku's house. While Goku is easily beating them in a playful fight, Goten and Trunks both get out of the drum and fuse into Gotenks and get back into the Barrel. The drum gets destroyed from the commotion as Goku turns into a Super Saiyan.
Negi Springfield bathes in an oil drum after getting some ninja training from Kaede Nagase. Kaede Nagase then Strips naked and gets into the drum with him. This seems to be Kaede Nagase's way of bathing during the times she is camping for her ninja training.
After Tatsumi Oga and Kaiser de Emperana Beelzebub IV go missing, they reflect that they took 10 different baths in a health club in the demon world, and one of the places they are shown bathing (besides a hot spring, hot tub, pond and waterfall) is a pair of oil drums.
Ryuuko Matoi takes a Bath in an oil drum at the Mankanshoku residence Off Screen. Triggers production report for this episode shows a sketch of Ryuuko actually bathing in the drum but the scene was cut out of the final episode for unknown reasons.
An unnamed female is seen bathing in an oil drum on the cover of a magazine while wearing a Towelon her head. The drum is somewhat transparent on parts due to the nature of it being on a lewd magazine.