Please create a new thread so its easy to help you on the issue.

I am thinking, you have 2 options

1. store the OAuth key and use it to make API calls. The decision to store the OAuth key depends on the number of outbound calls that would happen from source instance.

When there is a Invalid Key error is returned, then you make the request for a refresh token .

2. instead of storing the OAuth key, every time you make API call you make the OAuth token API first, use the retrieved token to tie it to your authentication header.