Can I get OSQL if I install SQL Server Client Tools?
The main recommendations appear to be:


If you have the install discs / files, then you should be able to install just the Client Tools.
Download SQL Server 2012 (SP1) Express (I would try the "SQL Server Management Studio Express (Tools only)" version first)
Check out: http://www.mssqltips.com/sqlservertip/1807/sql-server-2008-client-tools-installation/
If you already have a machine with OSQL installed, you might be able to copy just the following two files to the new server (place them in the same directory) and have it work:  


C:\Program Files\Microsoft SQL Server\110\Tools\Binn\OSQL.EXE
C:\Program Files\Microsoft SQL Server\110\Tools\Binn\Resources\1033\osql.rll



As far as licensing goes, I found the following file:

C:\Program Files\Microsoft SQL Server\110\Tools\Binn\Resources\1033\license_SQLCMD.txt

That has the following snippet:


  MICROSOFT SOFTWARE LICENSE TERMS MICROSOFT SQL SERVER 2012 COMMAND
  LINE UTILITIES
   ...
  1.    INSTALLATION AND USE RIGHTS. You may install and use any number of copies of the software on your devices to design, develop and test
  your programs.


Note:
OSQL has been superseded by SQLCMD. When you run OSQL it will display the following message:


  Note: osql does not support all features of SQL Server 2012.
  Use sqlcmd instead. See SQL Server Books Online for details.


However, if your existing code expects OSQL then you would need to test SQLCMD with your code first to make sure that any behavior changes between OSQL and SQLCMD do not break your code.