Edit your
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSDTC\MTxOCI keys to contain the following:
a) "OracleOciLib"="oci.dll"
b) "OracleXaLib"="oraclient9.dll".
c) "OracleSqlLib"="orasql9.dll"
Then reboot your server. (I'm assuming the Oracle linked server is Oracle 9i)
Victor