In Linux, the system call fork() creates a virtually identical copy of the running program.  Where there was one, there is now two.  The same program in the same spot.

Bitwise Or: |

Logical Or:  ||


4 in binary is 100
6 is 110.

4 | 6

1 0 0
1 1 0

1 1 0

6

Now all these flags for file openings have numeric values of powers of 2.

O_whatever is 1.
O_something_else is 2.
O_thatthing is 4.

O_whatever | O_thatthing = 5, or a combination of those 2 bits.