Homer’s Odyssey has a number of major themes, and readers will differ about which one is most important.
A likely choice is the centrality of loyalty and devotion. Odysseus sometimes strays from his dedication to these key values, but he never abandons them. He does not lose his resolve to return home to his wife and family and to lead his men home as well. Odysseus's heroism inheres not in his perfection, as he is all-too-mortal, but in his ability to regroup after a set-back and rededicate himself to his purpose of returning home.
Penelope's loyalty and devotion support the idea that this theme is central to the text. Despite his home being in disarray from the suitors' onslaught, when Odysseus returns, he finds that his wife has not only been faithful but also, in her way, as ingenious as he has been in order to achieve her goal of their reunion. Apart, neither of them could fully exemplify the ideal Greek life; together, they symbolize the best qualities of their society.
I would say the most prevalent theme in The Odyssey is the relationship between father and son and the inevitable maturation/development of both. Seeing the two men from Book 1 to 24 proves major changes and significant character development. Telemachus goes from boy to man, from very timid and shy to ready to rule the country while Odysseus himself goes from rash and arrogant to calculated and humble. Together, their relationship develops from estranged family to unstoppable team.