Login to your OnBuy Account, Go to the Listing & Products -> Imports & Integrations section
From this page access the OnBuy API section
From OnBuy API page, copy the values of Seller ID, Site ID , Consumer Key and Secret Key from Live keys section. Also check that the API integration service is enabled: it is enabled when the DISABLE INTEGRATION button appears with a red background
Now login to your OMS account and Go to OMS Marketplace Integration page and add those details in OnBuy Integration page.
Please open the ticket for your enquriy.