Is Nen related to Haki? I'm watching the Hunter x Hunter series and I'm also a fan of the One Piece anime. I notice that the power that exist in these two anime is somewhat similar.

I notice these similarities:


Ko in Hunter x Hunter is somewhat similar to the Armament Haki in One Piece which enables the part of a body to become much stronger.
Gyo and En is also similar to the Observation Haki in which enables the user to saw life energy, like Usopp saw, and detect/sense people around you within a certain range.
Ren and also En is likely similar to the Conquerors Haki which can knock down an intended opponent. In Hunter x Hunter, Ren and En did break some will of the characters in the series, i.e Killua.


If Nen is related to Haki, then is there a real world explanation for these techniques?
It would depend on your definition of related.

So although the two are similar, there are differences:

Armament Haki is close to Gyo, focusing aura on a certain part of the body. In HxH, characters can see/detect/sense people around them without gyo and en (an example of this would be Gon vs Gido, where Gon 'sensed' the spinning tops around him through concentration), however both techniques are used to detect concealed aura, through zetsu or In. 



Conqueror's Haki is a technique used for the purpose of knocking down people by dominating their will. In contrast, malicious intent behind strong aura will scare an enemy, intentional or not, like with bloodlust (Gon was scared of Hisoka's bloodlust in the Hunter exam arc, even though Hisoka was not aware of his presence). 

Ren exemplifies aura and with it the effect, and En means the person feels the aura around them, also with a similar effect. 

An explanation to their similarities could be that it is simply taking the most logical steps forward.

Life energy like things exist in many different series: aura in Hunter x Hunter, Chi in Dragon Ball, or Tenryoku in Beet the Vandel Buster. This is probably because it is an easy way of showing the strength of characters in a visual way. 

If there is life energy, then it would useful in a battle to be able to sense it. If it can be sensed, then there would be a technique to hide it. For dramatic technique, it could be used to manipulate an opponent far away. This results in similar techniques being developed in different series. Therefore, the power in One Piece and the power in Hunter x Hunter are not directly related. 

However, some of it could be related to real life: 

Human's are incredibly sensitive to other people's gaze. Even if you are not aware of it, you can tell when someone is looking at you, which results in the feeling of 'being watched'. This can lead to belief that one can 'sense' other people's presence. There are lots of tiny factors that can lead to awareness about someone's presence, such as sound from movement, or catching them in the corner of your eye.