William Trench has written a particularly strong OER textbook in this category, Elementary Differential Equations, which includes an optional chapter on boundary value problems. Trench’s text, like so many others, boasts a quality comparable to traditionally published books, not surprising given that was originally put out by a commercial publisher (as many OER math books are likewise born). It is fortunate that the author has made it freely valuable to all to learn and teach from. Other textbooks in this category that stand out from the rest are Notes on Diffy Qs by Jiří Lebl and Ordinary Differential Equations by Stephen Wiggins.