Some assumptions, but I guess it's hitting the nail...
- You've registered an account at odoo.com
- You've subscribed to Odoo Online
- You've got your own Odoo database for your own business (i.e. my-company.odoo.com)
Long story short, no, your odoo.com-account and your my-company.odoo.com-account are not the same. You can still use your odoo.com-account for logging in to your own database though. This can be done by utilizing odoo.com as an OAuth Provider, if you wish to do so.
What you need to make sure is that you first set up OAuth Authentication by navigating to Settings --> General Settings -> Integrations and activate OAuth: Authentication.
Next, go to Settings -> Users & Companies -> Users, open your user and then navigate to the Oauth-Tab. In here, select Odoo.com Accounts as your OAuth Provider and enter YOUR OAuth User ID. You can find your own user ID by referring to this recent post: https://www.odoo.com/forum/1/280667#answer-280848

Now you can use the Sign in with Odoo.com button at your database's login screen as well:
