details about open interst and its aplicacy u can get in some of archives at this forum.
what i like to add in short when open intrest price and volume all rising trend is firm and positive, when price and fall but volume and open intrest rises downside is strong, when price is constant volume and open intrest falls shows a reversal.
when to buy put, u can buy put when u are on short side of a trade. generally options call or put has to be bought when volatility is low, and to be sold when volatility is high.
u can use bollingner band, when it expands it is not the time to buy options call or put , when it narrows u can buy call or put based on ur stock view
generally time value eats away ur money. to trade in option best time is last week where time value normally is at par and u get quick results if u are right in judgement