When new files are created by programs in your account, they are inherently assigned your username as the owner, along with the default permissions for your account to access, which generally are full access for the owner, and then read-only access for the default group and everyone else.

The Finder in OS X is just another program that runs under the umbrulla of your account, and as such, is restricted to these same system permissions and file ownership restrictions.

When you copy files in the OS X Finder, first you will need read access to them, but the new file will be stripped of the old permissions and ownership, and inherit those from your account.

For the most part, such as when handling user data, this is perfectly reasonable since the new file copy will be fully accessible by your account; however, there are times when doing so where you may need the file to retain its original owner and permissions. One example is if you are using the “root” account to modify files in a user’s home folder, or if you are accessing files in the /Users/Shared directory, or in the /System or /Library directories.

These folders are likely accessed by users (including hidden system user accounts) other than your user account, so if you need to copy or replace them for whatever reason then you should preserve their permissions setups.

To do this in the Finder, when you have selected an item (or items) to copy, then press Command-C to copy them, but when you paste them in the new location, instead of pressing Command-V, include the Shift and Option keys as well and press Shift-Option-Command-V. You can also just hold the Shift and Option keys, and then select “Paste Exactly” from the Edit menu in the Finder.

When you do this, you will have to supply an administrator password, and then the copy will be completed.