OHLC data does not go back beyond a certain number of frames and depending on the interval it won't go back past 24 hours. The "since" value is mainly used to receive incremental updates.
You can build your own OHLC from trades data. Start with "since" of 0 to get trade data from the beginning.
Then from the returned data, grab the "last" time ID (i.e. ...,"last":"1383839436659595694"}})
Use that ID for the "since" value in the new poll to get the next set of data: