Opensips is a SIP proxy
Opensips Handles SIP Signaling and you can customize the routing via opensips scripts. There are also pre-configured scripts available to make routing for different scenarios.
Opensips have many modules to implement features like LCR, CDR, Dialog Handling, Dynamic routing, Fallback Mechanism, Load balancing etc
Using the module API, one can write customized script for routing SIP messages.
Media will be handled by rtpproxy or rtpengine
http://www.opensips.org/Documentation/Webinars
Freeswitch is a Softswitch or SoftPBX
Freeswitch is not just for SIP, It can bridge different VoIP Protocols and telecom Hardwares, Its a PBX system so it can also have features like Call Transfers, CDR, DID Routing, LCR, IVR, Conference etc
It also takes care of Media, Media transcodings
Its not mandatory to use Opensips with Freeswitch, both serves
different purpose. Opensips scales higher as it meant to act only on
SIP Signalling compare to Freeswitch. But it depends on requirement
whether Opensips is enough to suit your requirement.