Group Policy – Precedence of Conflicting GPOs in the Same OU

Screensaver policies are User settings, while you mentioned that the GPO is linked to the OU that the computers are in - the GPO settings in the User tree from that GPO wouldn't apply.

You'd need to enable loopback processing to get the desired result, but be careful with enabling it - it causes a second run of User policy processing against the computer's OU, and those settings take precedence over those that apply to the user's OU (or supplant them completely, if you run in loopback replace mode).

Local policies apply first; this means that their settings apply to the system, but are overridden by a setting from a linked domain GPO at any level.

Essentially, it's the lowest precedence location in which policies can be applied. You can set whatever you want, as long as no domain policy sets the same thing.

Take a gander here for lots more information on GPO processing order.