In my 11+ years of online teaching, I have had 1 student attend my Office Hours (also a 2 hours per week requirement). Students will find me online and send an IM, but the Office Hours are quite ineffective. I understand that ensuring a specific time to be available to engage with students is vauable. However, if the student were to contact me and set a specific time/date that worked for them, you achieve the same goal without wasting the instructors time.