OwO Lib adds no blocks, items, or mobs of its own, so a player who installs it and loads a world will see nothing new. That is expected, not a bug. The library only exists so that other mods can share code instead of each shipping their own copy.
Check the mod list, not the world
Open the mods screen from the main menu and confirm OwO Lib appears there. If it does, it loaded successfully. If a dependent mod is also installed and that mod's content appears in-game, OwO Lib is functioning — it is the invisible scaffolding underneath.
Watch for the failure case
OwO Lib matters most when it is missing. If you remove it while keeping a mod that depends on it, that dependent mod will usually fail to load, crash on startup, or show a missing-dependency error naming OwO Lib. That error message is the clearest proof of what the library does: it is a required shared component, not optional decoration.
What you should not expect
Do not look for an OwO Lib config screen full of gameplay toggles, and do not search creative tabs for OwO Lib items. If you want to see the library's effect, install a mod that lists OwO Lib as a dependency and use that mod's features. The library's job is done behind the scenes.