[X]: Poetry and art. While not a major focus of your studies, every young man of good birth is required to learn how to compose beautiful poems, which form the basis of nearly every form of formal social interaction, from courtship to seeking allies to simply connecting with friends.
[X]: The rituals of summoning and communing with spirits and ghosts. These allow a practitioner to speak with hidden or trapped spirits, and summon forth gods and spirits to help with a particular task, and even bind them as shikigami, a role somewhere between patron spirit and familiar. While communing with spirits is a necessary part of nearly every Onmyoji's life and work, the summoning of spirits is a tightly controlled practice, and misuse of it in the form of necromancy or other improper acts can result in severe censure from the Onmyoryo, the bureau of divination.
[X]: Chinese and the reading of chinese classics. While chinese is no longer quite so fashionable as it was a century ago, it is still an expected skill of young noblemen like you. Reading dusty chinese books on philosophy, ying/yang science, the five elements and buddhism is also a required and necessary skill in the study of onmyodo.
[X]: Calligraphy and writing. Writing beautifully is very important in heian society, and it is believed that a person's character and soul can be seen in their skill at calligraphy. A letter written in beautiful characters will always matter more than a letter that is merely functional; and a poem, ugly written, will be considered far beneath a lesser poem, beautifully written.
[X]: The preparation and blessing of ofuda and omamori, small paper talismans used for blessing, protection, good luck, sealing away and a number of other uses. Onmyoji may also use them as part of rituals of banishment, exorcism and other spells. This involves very carefully, and beautifully, writing the correct prayers and mantras on paper or silk talismans, and blessing them with rituals and the invocation of the right divine spirits and patrons. That said. The training itself consists mostly of writing the same prayers and mantras over and over and over again, focusing on their meaning, as well as studying the rituals of their blessing and empowerment.
[X]: The study of ying-yang theories and five elements lore. These are abstract, difficult and technical subjects, but are necessary for the most advanced forms of divination, and underlie and support all other subject an onmyoji is supposed to master. In practice, it requires a lot of reading and understanding of complicated texts on metaphysics and philosophy, and he use of sliding chinese instruments of divination, the full use of which can take a lifetime to master.