Yes, you can integrate with Odoo Cloud (Odoo Online) using their API, but there are a few things to keep in mind.

Odoo Online does support the standard XML-RPC and JSON-RPC APIs, so you can connect your existing C2C marketplace to it for things like CRM, payments, and invoicing. You’ll be able to push customer data, create leads, generate invoices, and even trigger payments if you're using supported payment gateways like Stripe or PayPal.

The main limitation with Odoo Online is that you can’t install custom modules. So if your integration needs custom fields or business logic, that could be a blocker. In that case, you might want to look into Odoo.sh instead — it's still cloud-hosted but lets you customize more freely.

If your needs are mostly standard (like syncing orders, invoices, and customer info), then yes, the API is usable and works pretty well for that. Just make sure to create an API user and authenticate via their JSON/XML-RPC endpoints.

Yes, but you need to pay for the "Custom" version of Odoo Online, not the "Standard"