Recently I had to make a connection to Oracle database from a Windows server platform. I need to make linked server in MSSQL Express.
I lost enormous time to due to some interesting Oracle client behavior.
The problem was hidden in some for first look unlogic requirement - despite my tnsnames.ora file does not contain any server name (only IP addresses) you anyway need to define that IP addresses in your hosts file. Otherwise your connections to Oracle will be unstable - sporadically getting error ORA-12545. Obviously Oracle client any way looks for server names and if it does not find, then in some cases causes that error above.
Thats what I got and that leed me in many wrong conclusions, configurations and time loss. But there also good things that I found:
Why some instant clients does not work and also good resource for Oracle instant client configuration in The Tendjee blog.
And custom cut down Oracle 10g client from David. That one I used finaly for my case.