The main considerations are:

- How are you going to upgrade
- How does the application function on 12c

The Database Upgrade Guide contains lots of good advice about how to do the migration. So ensure you read this!

https://docs.oracle.com/database/121/UPGRD/toc.htm

Also check this whitepaper on going 12.1.0.2:

http://www.oracle.com/technetwork/database/upgrade/upgrading-oracle-database-wp-12c-1896123.pdf

For application functioning, you'll need to test it as thoroughly as you can!

Do watch for the changes to the optimizer in 12c. Notably the adaptive features. You'll want to to test performance on 12c as best you can to ensure everything works smoothly. Read this whitepaper for more details:

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-optimizer-with-oracledb-12c-1963236.pdf

The Oracle Database Upgrade blog also has many posts about the migration posts. Here's a few you may find useful:

https://blogs.oracle.com/UPGRADE/entry/things_to_consider_before_upgrading
https://blogs.oracle.com/UPGRADE/entry/parameter_recommendations_for_oracle_database
https://blogs.oracle.com/UPGRADE/entry/parameter_recommendations_for_oracle_database1