Can I omit return from main in C?

Yes, as of C99, reaching the } at the end of main returns 0 if the return type of main is compatible with int.

5.1.2.2.3 Program termination

If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument;¹¹⁾ reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

Yes, the C99 standard says (§5.1.2.2.3):

reaching the } that terminates the main function returns a value of 0.