Althogh OPI does take your private data away from the large online cloud providers, there are still many built in features and apps in your phone that reports back to the OS/app provider with personal information and statistics.
Why not give ind.ie (https://ind.ie) a look. Their goal is to provide the possibility for a system based on open and free software, including a smart phone. They are launching on November 8th, join their thunderclap to help spread the word. https://www.thunderclap.it/projects/18064-ind-ie