Yes there is!
There are many freeware synthesizers whose express purpose is turning instructions like MIDI into sound. Currently it's far more common to support MIDI than OSC (I couldn't name one that supports OSC off the top of my head), but there's no reason why one couldn't exist.
Most of these "softsynths" tend to be released in Steinberg's VST format, in which they act as a "plug-in" to other hosting software, usually a Digital Audio Workstation environment like Ableton Live, REAPER, Pro Tools or similar programs. The host handles routing the MIDI data to the plugin and managing the audio output. There are other plugin formats too, like Apple's Audio Units, but VST is pretty pervasive and most hosts have some support for it. Likewise, there are also standalone versions of some synths that don't require a host, but I don't know of any free ones offhand.
Most DAW packages cost money, but there do exist some free ones such as Ardour. You can also, if you just want to play with your VST synthesizer but don't need the other features, find a standalone host that just loads the VST, nothing more.
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If you're on OSX, you can actually just use GarageBand, which should be preinstalled. It handles MIDI inputs and loading virtual instruments. If you don't like the included instruments, you can use Alphakanal Automat, a free AU synth plugin that GarageBand can host.
On Windows, I'm fond of Ichiro Toda's Synth1 VST plugin. It is also free, and you can host it in any VST host. The free evaluation version of REAPER will host it nicely and has the nice property of not being a time-limited despite basically being a demo.
There's more to MIDI than notes. Once you're all set up and happily playing your softsynths from your iPad keyboard, you might also want to be able to control the synth parameters from your iPad as well. See if your software supports sending MIDI continuous controller messages, which are analagous to knob/slider movements. Most synth plugins will respond to these, but there is some setup to map your CCs to the desired parameters. Check your particular host's documentation to find out how this is done in your host.