Neon Genesis Evangelion, The Big O Recommendations
If you like surreal shows about giant robots in a dreary, post-apocalyptic setting, ho boy, have I got the show for you.
Both shows take place in a post apocalyptic setting featuring mecha that are of god-like proportions. Both of them also feature psychological aspects and religious symbolism(although it only has meanings in Big O and not Evangelion).
Both series features science fiction themes with the involvement of mecha in combat warfare.
Both series features a style of old school art that are nostalgic. There is also action, drama, and some psychological themes especially involving some of the later episodes and plot.
Both series takes place in a post apocalyptic setting that also explores some religious themes.
Post-apocalyptic settings with philosophical issues and giant robots.
Both series are dark and surreal mecha anime, that deal with philosophical, psychological, and religious themes. Both start off as typical action-packed mecha anime, but later on become darker and darker. The ending to both The Big O and Evangelion are confusing and are up to interpretation.
Both are mech shows that, while they certainly stay within some of the same conventions of said genre, have a uniqueness to them which make them feel very refreshing.
Both are series that feature mechas and a surreal plot line.
Both shows have very similar mech designs, fight choreography, and characters. They also both have a similarly dreary tone and a psychological mind-bending plot. However, I feel that Eva gets a bit darker as it's story goes on, while Big O just gets more cryptic and enigmatic.