"Have occasion to" is a phrase used to indicate that a certain event or action has taken place. However, there are many other words and phrases that can be used as synonyms, depending on the context. Some of these include "had the opportunity to", "encountered", "experienced", "found reason to", "occurred", "arose", and "witnessed". Each of these phrases has a slightly different connotation and can be used to express different nuances of meaning. By incorporating a variety of synonyms into your writing, you can add depth and complexity to your language and explore the full range of possibilities available to you.