Show me your flowcharts [code], and conceal your tables [schema], and I shall continue to be mystified; show me your tables [schema] and I won't usually need your flowcharts [code]: they'll be obvious. -- Fred Brooks, "The Mythical Man Month"
In modern OOP, with code and data all mixed up and algorithms being injected at runtime with vastly different effects, things can get unreadable very quickly.
Mixing state between a bunch of different backing stores, or not making in memory data structures explicit, makes data structures harder to discover, but what does that have to do with OOP?