Gakuen Tokusou Hikaruon

Same OVA action approach, fast watch, a lone hero that transforms and fights evil. One is an "Investigator" and the other one is a "Detective" 

Both anime where release in the 1980's with similar animation and a similar vibe. The main protagonist in both titles can transform into a powerful armor superhuman being to fight against evil powerful monsters. The main protagonist in both anime also have a similar female love interest that they must protect from evil monsters. They both have plenty of action, violence and blood with a dark and gritty atmosphere. If you like one, you will enjoy the other.  

Both of them are "Henshin Hero" style anime (although Hikaruon is more based on the live action Space Sheriff trilogy and Metal Heroes franchise), i.e. each one has a lead character that transforms (Henshins) with a pose or device and a henshin call. Both also involve the lead character fighting off some sort of monster with their mecha style hero form.