In the bonus mini-game, Oni Spirits, Stage six has six enemies and six vases that must be destroyed to complete the stage; referencing 666, the Number of the Beast in Christian theology. This also self referential, as Onimusha is a series about defeating demons invading the Earth.

In the Japanese version, there's a pre-rendered cutscene of Hecuba revealing her true giant mantis-wasp like insect form to Samanosuke Akechi. In International versions however, the scene of her legs, in which her stinger emerges from between her legs, was cut.

In IGN's review of the game, a staff member wrote:
"A special note about Capcom's port to the US: For American gamers who don't get to play the Japanese version of Onimusha, you miss a pretty/nasty transformation scene that was totally hacked in the American version. The scene shows her laying down, spreading her legs and then transforming, quite sexually, into a giant flying wasp. The scene in the US version pretty much cuts out everything. I don't get it though. Onimusha is already rated M, and that scene shouldn't push it into AO territory, so what happened was that the scene was just butchered. Bad Capcom!"

The first Onimusha was originally planned to be released on the PlayStation in 1999 under the name Onimusha: Demon Warrior.

The strongest weapon in the game that Samanosuke can acquire is the Bishamon Sword. It is named after the Shinto God of War, Bishamonten.

On top of the elemental weapons that Samanosuke can acquire, he can also get two ranged weapons: A Bow (Yumi) and a Matchlock firearm (Teppo).
The bow and arrow was the traditional weapon of the samurai and had been in use in Japan since the ancient days. Samanosuke was also able to find and use Flaming Arrows, something that samurai armies liked to use against fortifications and buildings as they were made predominantly of wood.
The matchlock firearm, or Arquebuse, was introduced to Japan with the arrival of Portugese sailors and missionaries in the early 16th century. During the time that Onimusha: Warlords is set they were still not yet in vogue; although Samanosuke's enemy in the game (Oda Nobunaga) was one of the idea-men leading the charge to revolutionize samurai warfare with their integration into peasant conscript armies.

One of the three elemental weapons Samanosuke can use in the game is the Shippuu.
Shippuu translates to "Furious Wind" and is a wind elemental polearm. It is controlled by the Arashi Orb, which translates to "White Wind" orb.
It is patterned after a twin-sided voulge, similar to Serge's primary weapon in Chrono Cross.

One of the three elemental weapons Samanosuke can use in the game is the Enryuu.
Enryuu translates to "Fire Dragon" and is a fire elemental sword. It is controlled by the Koen Orb, which translates to "Red Flame" orb.
It is designed somewhat after a European broadsword.

One of the three elemental weapons Samanosuke can use in the game is the Raizan.
Raizan literally translates to "Thunderbolt Killing Blow" and is a lightning elemental katana. It is powered by the Shiden Orb. Shiden translates as "Purple Flash" referring to both a lightning bolt or as a euphemism for a sword's slash.
The Raizan is designed like a Katana.

In the original English release of Onimusha: Warlords the forces that aid Samanosuke are always referred to as "Ogres"; though in the Japanese they are called Oni. Future releases in the Onimusha series have always used the original Japanese term for them: Oni.

The main character, Samanosuke, while being based on real historical samurai Akechi Hidemitsu had his looks based off of actor Kaneshiro Takeshi.
Mr. Kaneshiro was also the Japanese voice actor for Samanosuke throughout the first three games of the Onimusha series.

Many of the demons in the game are named after characters in Shakespeare's Hamlet.
•Fortinbras was the Prince of Norway.
•Guildenstern was Prince Hamlet's friend and Courtier.
•Osric was a Courtier in King Hamlet's service.
•Reynaldo was Polonius' servant.
•Marcellus was one of the sentries who witnessed King Hamlet's ghost.