Osm2pgsql can be used in combination with ogr2ogr and a PostgreSQL data source.
An example command to export to GeoJSON would be
ogr2ogr -f "GeoJSON" roads.geojson -t_srs EPSG:4326 \
PG:"dbname=gis" -s_srs EPSG:3857 \
-sql "SELECT name,highway,oneway,toll,way FROM planet_osm_line WHERE highway IS NOT NULL"
Care should be taken if exporting to shapefiles, as characters may be present which cannot be represented in ISO-8859-1, the standard encoding for shapefiles.