Shuten Dōji (Demon Transformation)

In OVA 1 "a monstrous oni ogre disguised as the hero's female love interest reverts to his hideous true form," to quote Andréssaurio Rex, who found the clip.

Hell Teacher Nube (Oni Transformation)

In episode 25 has Nube's demon hand starts to act up; its seal breaks, causing his arm to completely transform:

 

Local yoko Tamamo examines the arm and says Nube's spiritual powers are at their weakest, and might allow the oni sealed in his hand to take over his entire body:

 

Sword of the Demon Hunter (Demon Transformations)

Thanks to ShareContent for recommending these videos on our Discord server. This anime features a transforming oni (demon), as you'll see in these clips from episodes 19 and 20. 

Red Earth (Monster Transformation)

A man transforms into a oni to fight against his opponent while a village burns:

 

Leo in his long journey transforms into a lion due to his curse advancing with every battle:

New Getter Robo (Oni transformations)

This is Daymon, presenting to you two transformation scenes from the 2004 anime New Getter Robo. The show was suggested for this blog several years ago by an anonymous commenter.

Here is the first scene:



And the other: