You can create and manage OAuth tokens in BigCommerce by creating an API account. Only the store owner can create API accounts. (Limit of 50 per store).
If you wish to create an API account:
1. Login to your store with your store owner's username and password.
2. Select Advanced Settings on the lefthand side panel.
3. Select API Accounts. This will take you to the Store API Accounts page.
4. Click Create API Account.
5. In the name field you can assign any name that you will recognize.
6. In the OAuth Scopes section, select the minimum scopes your site will require. There is more information about this here.
7. Click Save in the lower right-hand corner.
A new window should appear labeled BigCommerce API Credentials. Here your Client ID, Client Secret, and Access token will be displayed. There is no way to re-open this pop up window, so make sure to record the details, otherwise you will have to repeat the whole process.
Photos and instructions taken from the BigCommerce Support Page.