Not that I'm a phonics expert. I find it interesting and have learned a few things.
One answer how ORT is used in schools: in our school, ORT books are used as one of the reading schemes. There is a separate phonics curriculum and a separate writing curriculum.
Decodable books are books that include a limited set of phonics so that every word is accessible to a reader. Songbirds have a very clear progression that is the easiest to understand.
www.juliadonaldson.co.uk/songbirds.htm
In general, you can extrapolate that to the ORT levels, but there will be some inconsistencies. A decodable stage 4 book won't have split digraphs for example, and a decodable stage 2 book won't have long vowels.
This is a useful list of ORT books that I personally find more useful than the ORT website.
www.stmarysrcmidd.rochdale.sch.uk/files/files/ortbookbands.pdf
For the lower levels, in general, Songbirds, Floppy's Phonics, Decode and Develop, Snapdragons, and Glowworms are decodable. First sentences, Patterned stories, Traditional Tales, Fireflies, and In fact are less or not decodable. There are different series as the levels progress, and the question of whether or not a book is decodable becomes much less relevant after stage 5, when the alternative spellings have been introduced.