Using Oracle client to connect to remote database
There are different ways to connect oracle. One of them is easy connect .

You may use

sqlplus username/password@server/SID


According to comments. You can connect oracle using easy connect but hr (human resources) account does not exists. 
Therefore try to use

sqlplus username/password@10.66.1.61:1521/OPTPROD 


You should be able to connect your oracle system.