Write a SOQL query on FieldHistory (AccountHistory or OpportunityFieldHistory) to see who changed a specific field in the last 7 days.

Query the history object with a date literal filter and read who changed the field via CreatedBy.

SELECT AccountId, Field, OldValue, NewValue, CreatedBy.Name, CreatedDate
FROM AccountHistory
WHERE Field = 'Industry' AND CreatedDate = LAST_N_DAYS:7
ORDER BY CreatedDate DESC

For Opportunities use OpportunityFieldHistory (with OpportunityId). Field history tracking must be enabled on the field.