in TWS I can't even choose OPG as time in force (for nothing!)[both Mosaic and Classic] in the WebTrader I can choose OPG but when I try to buy MKT on an option chain of AAPL and set time in force as OPG I get this error: "OPG time in force is not supported for this order." any idea why? thanks in advance
You don't have to choose open, it would be added to the open automatically. I would not recommend market orders for the open, as there is often very wide option markets on the open.
It is my understanding that options markets can open right away after the stock is open at the primary. Most exchanges have their opening right away. These opening rotations are all automated now. As to "much time does it take for the market to balance after the open?", that would vary from option to options. The best thing to do is watch the opening for a few days, and you will see the markets start off wide then get narrower as time passes. The open is the worst time to find depth and liquidity for options.
You cannot get the opening price as the exchange does not support it. It's not up to IB. Spreads are high at 9:30, it's better to buy market after 9:35-36.
In one instance OPG orders are required. In the SPX, everyone is allowed to participate in the opening of the SPX options on VIX settlement morning (which will be tomorrow). The settlement price of the VIX will be based on the opening price of the July SPX options. In order to guarantee participation in the opening price of these July SPX options, you must mark the option orders OPG.