This is a good and appropriate question! There is much advice that says to always teach short vowel sounds first, but Oak Meadow introduces the long vowels first in kindergarten because long vowels sound like the letter’s name: A, E, I, O, and U. We introduce letters verbally first, with stories, songs, and verses, and the long vowel sounds are easy to hear. This is an auditory approach and a whole language approach (from the whole to the parts). It differs from phonics, which emphasizes decoding written words (from the parts to the whole).
We don’t do anything more formal in kindergarten. In first grade, long and short vowel sounds are introduced at the same time, beginning with letter A in lesson 1. We use a combination of phonics, auditory training, and whole language so students are coming at this new important skill of reading and writing from many directions, with a solid toolbox. We also don’t expect every student to be reading in first grade, so we can take a more rounded, gentle approach to reading. Phonics-based programs often include a lot of writing and decoding in the early years, so it makes more sense for them to introduce short vowels first because many short vowel sounds don’t need extra letters (short sounds like bed/hat/hop make good first words for decoding and sight reading, as opposed to long sound beet/field/play, which are tricky).
Here is an article that helps more clearly define and articulate Oak Meadow’s approach.