The Osmium Library has extensive support for all types of OSM
entities: nodes, ways, relations, and changesets. It allows reading
from and writing to OSM files in XML and PBF formats, including
change files and full history files. Osmium can store OSM data in
memory and on disk in various formats and using various indexes.
Its easy to use handler interface allows you to quickly write data
filtering and conversion functions. Osmium can create WKT, WKB,
OGR, GEOS and GeoJSON geometries for easy conversion into many GIS
formats and it can assemble multipolygons from ways and relations.

WWW: http://osmcode.org/libosmium/