So I was reading a book and in one part it said how some non-Christians in Bosnia preferred to be under ottoman occupation rather than being slaves under Christian occupation,. This is because it said that the ottomans allowed them to believe in any religion, is this true?
Yes, the Ottomans provided religious freedoms to a certain extent to what they considered in accordance with Sharia law. A perfect example of this is the Jews fleeing the Spanish inquisition to seek refuge in Ottoman Empire link.
For much of Ottoman history the territories were ruled by the millet system Where the various religious and ethnic communities can propagate their religion and culture granted it doesn't interfere with the central authority of the Ottoman Empire and they continue to pay jizya (special tax for non-Muslims)
This is why some communities (Albanians and Bosnians) chose to convert to Islam to reap the benefits where others (Serbs, Greeks, Armenians, etc.) chose to stick with their religion which was part of their national identity.