A little bit backwards - the trend is actually toward more O&O.
In the old days, networks used to be limited to 5 ‘owned and operated’ stations (later 5 VHF stations with a total of 7); now it is based on a percentage of the market - a network can’t own the stations in more than 39.5% of the market (with an adjustment for UHF).
Here is some Wikipedia (with the usual FWIW caveat):
http://en.wikipedia.org/wiki/Owned-and-operated_station
http://en.wikipedia.org/wiki/Owned-and-operated_television_stations_in_the_United_States
Thanks. :)