itachi is barely stronger. however, itachi is orochimaru's fated weakness, going by mythology. so he can never beat itachi.
that said, primarily orochimaru gets trolled vs itachi because of his overwhelming arrogance. that said, his fights vs itachi are way blown out of proportion. the first one was just a message, orocimaru could have and would have broken out of that genjutsu, but itachi showed him that possession was impossible. if he had wanted to fight, it would have taken nothing less than tsukuyomi or the totsuka sword to defeat orochimaru, and even then as we saw it would still not kill him. and having to resort to your strongest technique (which destroys your vision and pushes you closer to blindness), is not an easy win. and in the second fight, orochimaru was a great immobile creature w/o his own body ie easy prey to the totsuka sword, itachi's strongest last resort attack (which i still don't know how he didn't recognize if he'd been searching for it for ages, but whatever).