Own answer....
bson_init( &b );
{
bson_append_start_array( &b , "$or" );
bson_append_start_object( &b, "0");
bson_append_int( &b, "a", 4 );
bson_append_finish_object( &b );
bson_append_start_object( &b, "1");
bson_append_int( &b, "a", 2 );
bson_append_finish_object( &b );
bson_append_finish_object( &b );
}
bson_finish( &b );
Many thanks,
Álvaro.