Very weird considering that OSE usually has the smallest link count when AHREFs and Majestic tend to have a much broader data set (which is why if you are doing a link audit, you get as much data as possible from multiple platforms).
As James said above, it is more likely that OSE hasn't filtered their database to remove links that may have been deleted.
Out of curiosity, is the numbers above external links or referring domains? If it is the former, the removal of 1 site-wide link could mean huge fluctuations in link numbers
Cheers
Michael