Not from Companies House myself - but what is it you are looking to do e.g. are you aiming to automate filing? I believe that doing so via this API is extremely limited at present. Per documentation I believe you can only provide / update registered address, registered email and file some insolvency information. And the test (sandbox) for OAuth is further limited.
(You can search this forum also - use the magnifying glass icon at top right or e.g.:
https://forum.companieshouse.gov.uk/search?q=oauth
)
If you’re instead looking to allow clients to get data, much more can be achieved - and you don’t need OAuth access. If this is what you want, then see documentation for what’s available via individual queries or via listening for real-time updates.
Another option if you want a more full-featured approach there is the Companies House XML Gateway - but that service is not free.
Good luck.