Where can I get historic data series of Bitcoin prices?
You can download price data from bitcoincharts.com, in various time steps (although for the smaller time steps you an get only a couple of days at a time). But, las time I looked, they didn't list Huobi or OKCoin.
I downloaded from them several chunks of data about several exchanges in several epochs. Let me know if you you are interested.
You can download Huobi's individual trade data from https://github.com/huobi/btc-trade-result-history