Final answer:
In the episode 'Crime Aid' of 'The Office', Oscar Martinez discovers that Michael and Holly are dating after noticing their chemistry during a crime aid auction.
Step-by-step explanation:
In S5E4 "Crime Aid" of The Office, the character who finds out Michael and Holly are dating at the end of the episode is none other than the office's accountant, Oscar Martinez.
This revelation happens after Michael and Holly host a crime aid auction to recover items stolen from the office. Oscar notices the chemistry between Michael and Holly and concludes they are in a relationship, which is later confirmed by Michael himself.