Ressha Gattai Diesel-Oh is a mecha from the 38th Super Sentai series, Ressha Sentai ToQger (2014). ToQger is built around a railway theme, with the heroes fighting using "Ressha" (trains) that combine into giant robots. Diesel-Oh serves as the team's second, support-type robot, appearing alongside the primary mecha ToQ-Oh. Because ToQger was never adapted into a Power Rangers series, it has no US name and no corresponding Power Rangers toy — the mecha is Japan-exclusive.

Diesel-Oh is formed from three so-called Support Ressha: the Diesel Ressha, which becomes the torso, head and arms, plus the Tank Ressha and the Car Carrier Ressha, which form the legs. A defining feature of the whole ToQger line is the "Norikae Gattai" (transfer combination) gimmick: the five colored, numbered Ressha belonging to the core Rangers slot into the robot's chest and can be swapped freely between ToQ-Oh and Diesel-Oh, altering appearance and abilities. Diesel-Oh can also merge with ToQ-Oh and additional Ressha into larger combinations, with the Tank Ressha contributing a cannon arm.

The DX toy was released in 2014 by Bandai Japan as part of the ToQger toyline. It is made predominantly of plastic (ABS/PVC), as was standard for Bandai's Sentai DX mecha of that period; no significant die-cast content is known. The three included Ressha are assembled by hand into Diesel-Oh, and thanks to the line's shared connection standard the set can be combined with the separately sold DX ToQ-Oh as well as other individually released Ressha. It was never distributed through Bandai America or Hasbro and was sold only on the Japanese market.