ISAAC is a DRBG. It's like 1/3rd of a complete CSPRNG, and it's the easiest third to build. To use ISAAC securely on Linux, if for some reason you actually wanted to do that, you'd swap out the hash DRBG inside the kernel LRNG with ISAAC, and then continue using /dev/urandom.

Simply replacing the OpenSSL RNG or /dev/urandom in userland code with the ISAAC routines is likely to blow your app up.