One similarity between Odysseus and Arthur is that they are both kings who exist on that shadowy border between history and myth. Both fought in wars that are sometimes regarded as actual events from history, but they also have encounters with beings and creatures that are purely mythical.
We may also point out that both kings have groups of comrades who accompany them on their adventures, although the comrades of Odysseus who accompany him on his return voyage are much less well-defined than Arthur's famous Knights of the Round Table.
Both of these kings also have a famous weapon. Arthur has his sword, Excalibur, whereas Odysseus has a bow, which only he can string, with which he kills Penelope's suitors.
Both men encounter magical beings: Odysseus encounters Circe, while Arthur has dealings with Merlin. Interestingly, whereas Circe turned Odysseus' men into pigs, one legend associated with Arthur has him searching for a wild pig.
Both kings have famous wives who experience sexual temptations. Unfortunately for Arthur, Guinevere gave in to those temptations with Lancelot.
Both kings make journies across the sea to mystical lands. Whereas Calypso and Nausicaa cared for Odysseus after he was shipwrecked on two different occasions, Morgan Le Fay is said to have healed Arthur after his fight with Mordred.