RyanJP wrote:(...)so Octane is already spectral based and doesn't need OCIO
It doesn't need it to produce appropriate and subjectively aesthetic images (
this page takes a broader approach to this aspect), but it does for a "full ACES pipeline", as mentioned on the linked page. There are unfortunately no shortcuts to those deep topics, be sure to "read between lines" to avoid any misunderstanding.
Spoiler: it's needed for texture level color space operations and to properly conform during pre-viewing (IPR). The ACES checkbox is basically the ACES RRT+ODT (Output-sRGB).RyanJP wrote:(...) without OCIO/ACES does it still have the same range in the color gamut as ACES when exported?
During "rendering calculations", it does. During EXR (or JPG or whatever) encoding, it no longer does as it default to sRGB (non-linear or linear, depending on the chosen file format). However, there are conventional ACES color spaces natively available, so the option to encode to ACES (AP0 or AP1) is offered.