We don't have an updated crosswalk showing all possible standards templates, fields, and their relevant XML fields - we used to maintain spreadsheets like this, but they became very difficult to maintain over time with so many different content standards and export templates. However, I can provide you some links that should give you most of what you need, and a suggestion on how to find the rest.
First, the data entry pages have some of this information. For example, you can find the ISAD template EAD XML mappings on this page:
However, the DC XML fields are not crosswalked here. You can see the DC XML mappings on the DC template data entry page:
I will admit that while I don't think anything major has changed, there have been a couple releases since I have exhaustively gone through all these pages and confirmed they are fully up to date, so please keep this in mind.
In light of this, one of the best ways to create your own crosswalk is to import one of our sample CSV templates, and then export it in the desired XML format. I've added the relevant ISAD number and field name in each sample CSV template, so when exported as XML, you can easily see how they are rendered.
I've gone ahead and done this in my local test Vagrant instance, and am attaching the results for you, using the ISAD CSV template as the basis.
There's not an easy way in the UI to to get the OAI identifier of a record, but you can use the following SQL query:
Replace your-slug-here with the actual slug of your target record.
For information on how to access the MySQL command prompt, see:
The actual full identifier you will use in an OAI request depends on your OAI settings, as it combines the configured site base URL, OAI repository name you've configured, and the unique OAI identifier automatically generated by AtoM. So for example, in my test instance, though the SQL query above just returned a value like 12345, the value I used in the OAI Get Record query was 10.10.10.10:atomdemo_12345. You can see the Get Record OAI query format here:
I hope this helps!