Yahoo finance: are the open prices adjusted for dividends?

For the raw data, the open, high, and low price are not adjusted. They are they actual prices at which the stock trades pre and post dividend.

The adjusted close price is just what it says, an adjustment of the close, done so to take into account all distributions so that one can calculate the total return on the investment.

The exception to this is data for charting programs. In this case, all of the data prior to the special dividend is adjusted so that there isn't a large gap on the chart, destroying the ability to do technical analysis.