yum yum - [Errno 4] IOError: <urlopen error>

It's not something new with message "[Errno 4] IOError: <urlopen error> ". When you found with this error message. You might regenerate the metadata cache again. For my issue, 'Yes'! it's fixed after doing it.

[root@linux yum.repos.d]# yum update yum
Loaded plugins: rhnplugin, security
This system is not registered with ULN.
ULN support will be disabled.
http://public-yum.oracle.com/repo/OracleLinux/OL5/9/base/x86_64/repodata/primary.xml.gz: [Errno 4] IOError: <urlopen error>
Trying other mirror.
Error: failure: repodata/primary.xml.gz from ol5_u9_base: [Errno 256] No more mirrors to try.
[root@linux yum.repos.d]# yum clean all && yum clean metadata && yum clean dbcache && yum makecache
Loaded plugins: rhnplugin, security
Cleaning up Everything
Loaded plugins: rhnplugin, security
0 metadata files removed
0 sqlite files removed
0 metadata files removed
Loaded plugins: rhnplugin, security
0 sqlite files removed
Loaded plugins: rhnplugin, security
This system is not registered with ULN.
ULN support will be disabled.
el5_latest                                                                                                               | 1.4 kB     00:00
el5_latest/filelists                                                                                                     |  21 MB     04:06
el5_latest/updateinfo                                                                                                    | 402 kB     00:03
el5_latest/other                                                                                                         | 103 MB     18:23
el5_latest/group                                                                                                         | 1.1 MB     00:10
el5_latest/primary                                                                                                       |  13 MB     00:09
ol5_u9_base                                                                                                              | 1.4 kB     00:00
ol5_u9_base/filelists                                                                                                    | 3.1 MB     00:28
ol5_u9_base/updateinfo                                                                                                   | 400 kB     00:03
ol5_u9_base/other                                                                                                        |  12 MB     02:16
ol5_u9_base/group                                                                                                        | 1.1 MB     00:14
ol5_u9_base/primary                                                                                                      | 1.2 MB     00:11
el5_latest                                                                                                                          10787/10787
ol5_u9_base                                                                                                                           4688/4688
el5_latest                                                                                                                          10787/10787
ol5_u9_base                                                                                                                           4688/4688
el5_latest                                                                                                                          10787/10787
ol5_u9_base                                                                                                                           4688/4688
Metadata Cache Created
[root@linux yum.repos.d]# yum update yum
Loaded plugins: rhnplugin, security
This system is not registered with ULN.
ULN support will be disabled.
Skipping security plugin, no data
Setting up Update Process
Resolving Dependencies
Skipping security plugin, no data
--> Running transaction check
---> Package yum.noarch 0:3.2.22-40.0.1.el5 set to be updated
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================================================================================
 Package                     Arch                           Version                                    Repository                          Size
================================================================================================================================================
Updating:
 yum                         noarch                         3.2.22-40.0.1.el5                          el5_latest                         1.0 M

Transaction Summary
================================================================================================================================================
Install       0 Package(s)
Upgrade       1 Package(s)

Total download size: 1.0 M
Is this ok [y/N]: y
Downloading Packages:
yum-3.2.22-40.0.1.el5.noarch.rpm                                                                                         | 1.0 MB     00:00
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Updating       : yum                                                                                                                      1/2
  Cleanup        : yum                                                                                                                      2/2

Updated:
  yum.noarch 0:3.2.22-40.0.1.el5

Complete!
[root@linux yum.repos.d]

Reference:
http://yum.baseurl.org/wiki/YumCommands