Thanks to @mseri, euler is now on opam! And in the meantime it has gained more functions, the most striking of which being Primes.prime_seq : int -> int Seq.t (thank @cuihtlauac for pointing me to the algorithm!) and Arith.smallest_root : int -> int * int (the latter in the next release, 3.0, not on opam yet).
Also, since I still cannot edit the first post, this is a reminder that the project has moved to GitHub: repo, doc. The links in my first message won’t be updated.
Perhaps? I am not very familiar with Owl (ask Owl’s Benevolent Dictators For Life? 
). Euler is an integer arithmetic library whereas, as I understand it, Owl is mainly a floating-point math library for scientific computing (linear algebra, differentiation, statistics and and such). I would guess that prime numbers and friends are rather remote concerns for Owl users, but perhaps Owl is interested in expanding its domains? I did spot a small number of integer arithmetic functions in Owl, most of which are also found in Euler.