The ocarina is half an instrument, and the other half is your body. I separate body skills into two branches: external and internal. External means that the action can be seen from the side and corrected easily. And internal means that actions occur inside of the body and can’t be seen.
External body skills:
Fingering. The actual fingering of the melody. Which holes are closed and opened.
Hand and Finger Position. The angle of wrists, how far fingers are from holes.
Posture. The position of the whole body. Open shoulders and straight spine. You can read more in the article 'How to hold an ocarina' on pureocarinas.com.
Internal body skills:
Breath control. Using correct breathing technique, strength of the breath, breath pressure, vibrato.
Tongue control. Position of tongue. Tongue articulation
Knowledge based skills
Skills that can't be acquired naturally through playing the instrument without additional study.
Reading music. Pitch and length of notes, keys and accidentals.
Music theory. What makes music to sound like music. Scales and intervals. Harmony.
Theory of the instrument. Physical limitations of the instrument and how it works.
Practical Music skills
Skills that can't be learned without practice and performing.
Aural Skills. Finding music by ear. Hearing mistakes. Learning from the performance of other players.
Intonation. Your ability to stay in tune with yourself and with other instruments.
Rhythm. Your ability to start and end notes in time.
Transposition. Changing any melody to fit your ocarina's range.
General musicality is your ability to understand the language of music and use it to tell the story you want to tell.
Phrasing. Separating a melody into 'musical sentences' to form a complete idea.
Ornamentation. Adding interest and complexity to music.
Expressing Emotions Articulation of important parts of music.