There are, of course, multiple themes within Homer's Odyssey, as it is a very long epic poem. However, there are a few that are more prominent than others:
The first and foremost is the concept of xenia (ξενία, pronounced "zen-EE-uh"), which translates as "guest-friendship." This is the concept of hospitality, which is ruled over by Zeus, showing just how important it was to the ancient Greeks. I have included some links that further explain the concept of xenia not just in the Odyssey, but as a general concept.
Another concept is sophia (σοφία), which translates as "wisdom" or "cleverness." Odysseus is not incredibly strong or skilled like Ajax or Achilles, but he is often described as wily and crafty; the Odyssey is very much about brain over brawn. One example of this is how Odysseus tricks the Cyclops and accomplishes his escape.
A third concept is agalma (ἄγαλμα), which translates as "glory." This is a bit less important than the other two, but it is still there. Odysseus has gained much glory from the Trojan war and his exploits.
The last concept I will talk about is pistos (πιστός), which translates as "trustworthy" or "loyal." One major example of loyalty is Penelope, Odysseus' wife, who holds off her suitors for the entire time Odysseus is gone despite their constant harassing.
These are by no means the only themes within the Odyssey, but they are some of the more important ones.