You already know that GPOs get applied in order of the tiers at which they are linked, Local | Site | Domain | OU. Occasionally, GPOs linked at the different tiers could conflict with each other or contradict each other, and the last settings applied win. OU-linked policies will trump Domain-linked policies, Domain-linked beat out Site-linked, and so on.

A great resource for discovering these potential conflicts is the Group Policy Inheritance tab inside GPMC. This tab will show you all of the GPOs that are going to attempt to apply to the particular location you have selected, not only for the selected OU but for all tiers above it. For example, in the following screenshot, I have clicked on my Testing GPOs OU and can quickly see all GPOs that are going to attempt to apply to any workstations or users inside this testing OU. Some of these GPOs are linked straight to my OU, and some are linked at the root of the domain:

You might wonder why I keep using the word attempt. When a GPO is linked, it is linked, right? Aren't all of the GPOs listed in the inheritance tab going to apply to my machine? Not necessarily. Group Policy Inheritance is showing you what policies will apply based upon their links, but does not consider other factors. Some of these GPOs may still fail to apply to you based on Security Filtering, WMI Filters, or any other GPO-specific filtering criteria.

Occasionally, multiple GPOs will be linked at the same tier, and also conflict with each other. Once again, the Group Policy Inheritance tab can help resolve this issue. Looking back at the preceding screenshot, look closer at the Precedence column. These numbers indicate the priority of the GPOs, which are linked to this location. If you notice something out of whack here, you can pop back over to the Linked Group Policy Objects tab and use the arrows to adjust GPO precedence order, thereby adjusting the order in which the GPOs are applied, which affects the winning settings at the end of the Group Policy processing cycle.