But if you *do* invest -- only buy stocks that are going up. (That's my advice, anyway. I mean, you know, "YMMV" and all of that...)
High open interest means that there is....interest in the underlying. If it is long or short interest depends on the trade, i.e. if it was on the bid or the ask. To know this you need to have daily times and sales. Also, as someone already reported, these transactions can be part of a bigger trade or even hedging of an opposite direction stock trade.
And after hedging long stocks by selling S&P futures, if they are very good at hedging and market timing, they can sell twice as many S&P futures looking for overall profit and hedging that position with options. Some would ask why not just dump the stocks, if you been sitting on them since 2009, you will incur a needless tax situation next year, plus in meantime dancing option plays around the stocks would have to be unwound, and loss of dividends. So just easier to hedge with futures.