I am working on a project that uses Oracle 11g, and we were standing up a db for proof of concept in one of the dev environments. I configured listener.ora to point to the instance, but still was having connection problems.
Apparently in your init file, you need the line:
local_listener='MySidName'
To make sure it keeps loading, run the following via sqlplus:
C:\> cd some_path_with_my_init_file
C:\> sqlplus /nolog
SQL> connect / as sysdba
SQL> shutdown immediate
SQL> startup pfile='myinit.ora'
SQL> create spfile from pfile
Hope this helps someone…