As of 14 July 2009, Oracle is now introducing a new method for patching, patch set updates, or PSU. According to MOSC notes 854428.1 and 850471.1, Oracle PSU is a new patching strategy whereby the DBA can choose only "recommended" and "proactive" patches, instead of all of the patches in a quarterly Critical Patch Update (CPU).
An Oracle PSU contains recommended bug fixes and "proactive" cumulative patches, a nice change that makes it simple for the DBA to choose to apply "priority" patches.
The Database Patch Set Updates and Critical Patch Updates that are released each quarter contain the same security fixes. However, they use different patching mechanisms, and Patch Set Updates include both security and recommended bug fixes. Consider the following guidelines when you are deciding to apply Patch Set Updates instead of Critical Patch Updates.
• Critical Patch Updates are applied only on the base release version, for example 10.2.0.4.0.
• Patch Set Updates can be applied on the base release version or on any earlier Patch Set Update. For example, 126.96.36.199.2 can be applied on 188.8.131.52.1 and 184.108.40.206.0.
• Once a Patch Set Update has been applied, the recommended way to get future security content is to apply subsequent Patch Set Updates. Reverting from an applied Patch Set Update back to the Critical Patch Update, while technically possible, requires significant time and effort, and is not advised.
• One-off patch is specific bug fix for requested customer.
There are critical and DB patches are there and it’s applied to Oracle binaries, DB dictionary and agent services. If you are using RAC patches are available for cluster services also. Mostly it will be consolidated in critical patches which will be released every quarter. You need to have metalink access to download the patches and corresponding document. You need to read and understand carefully this document before attempting applies patches. You may try the patches on testing environment before applying in live.
Patch will be released in January, April, July and October on each year. Doc Id: 1291877.1