Choosing the best ocarina key for a tune
While the ocarina is fully chromatic, the restricted range of a single chamber severely limits the keys which can be practically played. You get an octave with three notes above and—if your ocarina has subholes—one or two below. This limits you such that, if you play in D on a C ocarina, you actually lose not one but two diatonic notes from the top of the range. You can play up to the high E, but the next notes in the scale—F♯ and G—are out of range.
Now you could transpose the music to fit, which is fine if you're playing alone or if your accompanist can also transpose with you, but this isn't always practical. You sometimes have to play in a given key in different situations, such as to fit with a vocalist, a pub session (where, in the UK, the keys of G and D are standard), or a non-midi backing track.
When I first started playing the ocarina, I just transposed things to fit without paying much notice to the key. Because of this, I unwittingly played in uncommon keys. As I developed as a musician and wanted to play with accompaniment, I consequently couldn't find anyone willing to play with me. For this reason, I strongly recommend learning to play pieces in their original or most commonly played keys so you can more easily play with others.
Not every piece will fit within the range of your C ocarina. So how do you deal with this? If the piece you want to play has only one or two notes out of range, you may be able to replace those notes with others from their respective chords. But if you want to be able to play in most situations, you'll have to go beyond your C ocarina.
Here's a straightforward example: the Swallowtail Jig in A Dorian. A quick scan over the notes reveals that the tune has a range of G to high B, so it won't fit at all on a C ocarina in this key. The solution here is to play it on a G ocarina. As the F♯ is present natively on such an ocarina, no accidentals are required.
Now take a look at the following tune, Brighton Camp, which isn't so straightforward. Looking at the key signature, you can see that the tune is in G. Scanning over the notes, however, will reveal that it has a range of low D to high G—too low for a G ocarina and too high for a C. As most of the character of this tune is on the high end, you could opt to modify the low notes to fit on a G ocarina. A better solution is to play the tune on an ocarina in D, replacing the instrument's native C♯ with C natural as an accidental.
Just because a tune is in a given key—C, for example—doesn’t mean that it actually fits a single octave starting and ending on the tonic note, i.e. C to C. It's more important to look at the note range than the key when selecting an ocarina. Tunes with the key note in the middle of the range aren't uncommon.
Another such oddity is a tune called Lemmy Brazil's no 2. What's the key? D Major. What's the range? F♯ to high A. How can you handle it? Play it on a 11+ hole G ocarina. Since the F is already sharp on a G ocarina, you only have to add the C♯, which is fingered the same as an F♯ on a C ocarina.
What keys of ocarina you need depends on the music you play. For the folk music I'm involved with, the most useful ocarinas are G and D. If you don't have a suitable ocarina, it's also possible to learn a tune such that it would be in the correct key if you did. For example, you might want to play a piece suitable for a Bb ocarina but not have one. If you were to play it with the same fingerings on another ocarina as you would on the Bb, it would come out correctly on a Bb instrument when you get one.
All of the above tunes could be played on a C ocarina, keeping exactly the same fingerings as on the recommended keys above. But they would come out in different keys—namely D Dorian, F Major, and G Major.