Its fairly stright forward to implement, via OneTrust dashboard generate the snippet and place within the head section of your pages (towards the top as possible). Most of our solutions we have the ability to add scripts via the CMS that filter down to all pages so makes life easier to add.
I recommend adding the snippet directly to your pages rather than using GTM as it must be loaded before any other scripts or tags on the website (to ensure that user consent is properly managed). If implemented through GTM, there is a risk that it may not load in the correct order, which could result in incorrect or incomplete consent management.
We will be integrating with the One Trust cookie consent banner to manage the cookie requirements. I'm reaching out to see if anyone here has had experience in the past integrating Optimizely with One Trust, and if there are any recommendations, insights or challenges you should share from that experience.