In The Alchemist, are omens just illusions to justify something we feel or want, or are they something more?
Omens are presented as being much more than illusions to justify something we feel or want. The narrator of this tale presents omens as being an essential part of how the Soul of the World communicates with those who are open to its voice. Note what the Englishman says to Santiago when they meet:
Everything in life is an omen... There is a universal language, understood by everybody, but already forgotten.
Omens, whether it is in the form of the Urim and Thummim that fall from the pocket of Santiago, or the vision of the hawks that he has in Al-Fayoum, show Santiago that he is on the right track and he is doing what he should be doing and going where he should be going. Omens therefore form a part of the Universal Language of the World, and Santiago, when he is able, receives encouragement and support through discerning omens and identifying them for messages of confirmation.
At the same time, omens are also used as a kind of marker to represent Santiago's increased maturity and growth in the story. This can be seen in the way that the omens develop from being very individual, small signs to becoming much bigger in scope that involve more than just Santiago's existence and future. This can be seen in the vision of the hawks and armies mentioned above, at Al-Fayoum. The greater the omens indicates how close Santiago is on his personal quest to understanding the Universal Language of the World and achieving his Personal Legend.