convert angles to radiants

I want to rotate an object 90 degrees because doing Mathf.PI / 2 me it always turns 180 degrees?

Code (csharp):

1.  
2. ogg.transform.rotation.z = Mathf.PI / 2;
3.  

 

transform.rotation does not use radians, it is a Quaternion
http://docs.unity3d.com/ScriptReference/Quaternion.html
You should never directly set the x,y,z and w of a rotation.

As for actually converting degress to radians, Mathf has consts to do this
float degrees = 30;
float radians = degrees * Mathf.deg2rad;

 

so there are ways to rotate the GameObject with radians?

 

First you need to pick the correct method. There are many that can be used for rotations:
http://docs.unity3d.com/ScriptReference/Transform.RotateAround.html
http://docs.unity3d.com/ScriptReference/Transform-eulerAngles.html

Or if you want to work with quaternions, you may use:
http://docs.unity3d.com/ScriptReference/Quaternion-eulerAngles.html
http://docs.unity3d.com/ScriptReference/Quaternion.Euler.html

All those methods work with degrees. If you want to store your values as radians, that's not a big deal, you just need to multiply them when you get and set them as mentioned by @hpjohn.