If you have ordered parts via BrickOwl, you can import your order directly into your My Parts list on Rebrickable with the following steps:
1. On your My Parts page, click the 'Import from BrickOwl Orders' link.
2. To get a BrickOwl API key (a one-time process), go to your BrickOwl profile
page and click the API Keys tab. Then click the 'Add API Key' button.
3. Enter your key into the import box on Rebrickable and click the 'Get List of Orders' button.
4. Select the appropriate order and click the 'Append' or 'Replace' button as appropriate. In the future, the page will remember your key so you don't have to enter it in again.