Can I use a Cloudflare Origin Certificate with a domain that isn't using Cloudflare's proxy?

No. The primary purpose of the Origin Certificate is to secure the traffic *between* Cloudflare and your origin server. If your domain isn't proxied through Cloudflare, the Origin Certificate serves no function and is therefore irrelevant. You'd instead use a standard SSL/TLS certificate issued by a public Certificate Authority (CA) to secure traffic directly to your server.