Oracle 12c R1 is the most current release certified with R12.6.
You can view the compatibility matrix after you log into support.ca.com here:
Harvest Software Change Manager r12.6 Compatibility Matrix - CA Technologies
Attached are some documents relating to problems that Level 1 support has found in testing
Harvest with Oracle 12c.
These are not official documents by any means but they are documented problems which
may help the customer going forward with Oracle 12c.
L1 has found a show stopper problem with Oracle 12c (188.8.131.52) that check in/out fails
with a datetime format error from Oracle. Customers that have installed and have run
into this problem have downgraded to Oracle 184.108.40.206 and have not had any further problems.
05 APR 2016 - ADDENDUM: Other customers have reported that leaving the Oracle 12c server at 220.127.116.11
but downgrading the Oracle Client driver to 18.104.22.168 circumvents the problems of the show stopper
problem as described above.
20 JUL 2017 - ADDENDUM: Level 1 support tested the Oracle 12c server and client 22.214.171.124 with Harvest V13
and found that the show stopper error of invalid datetime format error still exists with this Oracle version. The solution
is to install the 126.96.36.199 Oracle client driver (32bit or 64bit) to connect to the 188.8.131.52 Oracle server.