There's absolutely absolutely no bias in oanda's boxoptions. If you draw a box 200 pips over and a box 200 pips below px it's going to have precisely the identical premium (marginally distinct due to carry on longer term options for some pairs). The price of a boxoption is a function of time and volatility. Oanda's house edge is put by a reduction in payout/increase in box price (based on fair value) - in precisely the exact same way like there is a spread for trading.