OPatch was not able to get details of the home from the inventory

If OPatch is failing becuase of inventory issues, you can force it to use the inventory you have. Try to use this:
-opatch apply -invPtrLoc

Server Level and Application Level Configuration Files in OAS

I was editing, using, sometimes digging these files but it is really useful to have them all listed like this.
I found it really useful especially when you knew to edit this xml file but not sure where it was or how to do it.
Here is the documentation from Oracle Technology Network Documentation:
http://download.oracle.com/docs/cd/E12524_01/web.1013/e12288/confdesc.htm
Enjoy!

Install has detected that Oracle Application Server is already installed in this Oracle Home

If you are trying to install SOA Suite(might be just Oracle Application Server) and you get this error even though the home you are trying to use is not being used, then most probably the orainventory file has been corrupted.

-You can try to check whether this home is listed on the file /oraInventory/ContentsXML/inventory.xml. If so try to remove it and reboot your machine. Then rerun the installer.

- If this does not help, you can force the installer to use another inventory file that you create. I.e:

• Create a file called oraInst.loc and add these lines:

inventory_loc=/u01/app/oracle/oracle/product/10.1.3/oraInventory
inst_group=dba



• run the installer with this parameter:

runInstaller -invPtrLoc /u01/app/oracle/oracle/product/10.1.3/oraInst.loc