Why does null reference print as "null"

It might help showing you the bytecode. Take a look at the following javap output of your class:

> javap -classpath target\test-classes -c RefTest

Compiled from "RefTest.java"
public class RefTest extends java.lang.Object{
public RefTest();
  Code:
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   aconst_null
   1:   astore_1
   2:   aconst_null
   3:   astore_2
   4:   getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   7:   aload_1
   8:   invokevirtual   #23; //Method java/lang/Object.toString:()Ljava/lang/String;
   11:  invokevirtual   #27; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   14:  getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   17:  aload_2
   18:  invokevirtual   #33; //Method java/io/PrintStream.print:(Ljava/lang/Object;)V
   21:  return

}

Just looking at the main method, you can see the lines of interest are where Code is 8 and 33.

Code 8 shows the bytecode for you calling o.toString(). Here o is null and so any attempt on a method invocation on null results in a NullPointerException.

Code 18 shows your null object being passed as a parameter to the PrintStream.print() method. Looking at the source code for this method will show you why this does not result in the NPE:

public void print(Object obj) {
    write(String.valueOf(obj));
}

and String.valueOf() will do this with nulls:

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

So you can see there is a test there which deals with null, and prevents an NPE.

System.out.println(o.toString())

o.toString() is trying to dereference a null object to convert it to a string, before passing it to println.

System.out.print(o1);

The print being called is the print(Object) variant, which is itself checking that the object is not null before proceeding.