Many times FII also make use of Option Buying for Hedging their positions that they have taken in Cash Markets so as to mitigate the losses.Hence coupe of times OI can also have mixture of writing+buying.If it would have so easy to determine whether its writing or buying then money making wont be that much tedious job in markets.
Every time an option is written a fresh contract gets added so OI has to go up.
An option may be bought from a writer (OI going up) or from an earlier buyer (OI remains same) or it can be bought back by the writer (OI going down as the contract is extinguished).
So it is safe to say that if OI is going up - it is writing and if OI is going down it is buying. I am afraid that money making still remains difficult.
regards.
rm