For anyone new to OSC, liblo (http://liblo.sourceforge.net/) fits the bill for a lightweight C implementation whose initial commit wasn't 8 hours ago.
Oh hey! Funny to see an OSC implementation called "tinyosc" that isn't the one I wrote in Rust a month or so ago [0]. ;)
In all seriousness, you could also look at fundamental's rtosc[1], which is fairly minimal as well but has been around for a while and is being used in the ZynAddSubFX synth.