Well actually....

....the original author/s of the Hippocratic Oath would certainly have assumed that slavery was ok thus including the concept of it within the oath.

The Bible too - both OT & NT - also accepts the reality of slavery as a given.
No admonisions against slavery per se are ever made though scripture does urge people to treat their slaves justly and even with love.

It would take human society well over 2000 years (the Oath is commonly assumed to have been penned in the 4th c. BCE) to react against the concomitant evils of slavery and actively destrot it as an institution.