Oh what a difference a half-point makes.

Saturday's Denver-Pittsburgh game opened with the Steelers favored by 6 1/2 points in several Las Vegas sporting houses. Professional money showed immediately for Pittsburgh, and the spread quickly went out to seven. It will not be coming back in.

Pittsburgh is still my pick, but for an imaginary $250 instead of the $500 or $1,000 that might have been risked had the number held at less than seven. Such is life.

The Steelers beat up the Broncos and Craif Morton two weeks ago at Denver.They led at the half, 21-0, then rested Terry Bradshaw and other key players over the final 30 minutes as substitute quarterback Norris Weese brought Denver within four points and one yard of a winning touchdown at the finish.

Weese is the only hope the Broncos have against the Steelers-and his passing is erratic. Morton does not figure to be able to stand up to the pressure mounted by Pittsburgh's passrush. Denver's excellent defense will be hard-pressed to more than hold its own against Bradshaw, Lynn Swann, Franco Harris and friends.

There is a decided edge in this match-up, starting with the line play. Everything points in Pittsburgh's favor. with Weese a longshot at best to reverse the momentum in the second half.

There is one other consideration: weather and its effect on field conditions. Denver would benefit from an off track. So, if the game is to be played in a Winter Wonderland cut the risk to an imaginary $125 on Pittsburgh.

Field conditions also could be a factor, in Foxboro, Mass., Sunday when Houston invades New England. The Patriots are looking for fast going. Houston and Earl Campbell would move up in a blizzard.

I'll take New England giving six points for an imaginary $250 if it's dry; $125 if it's wet.

Dan Pastorini turned in the best game of his career last weekend in Miami. I doubt he can come close to repeating such an effort.

New England, meanwhile, will be moving much more effectively on the ground than Miami did and Steve Grogan, inept as he is at times, can hardly have a worse afternoon than the injured Bob Griese did.

The Oilers will invite Grogan to throw. They will jam their defense against the run and hope Grogan has one of his bad days. It is their only chance.

I'll also string along with Dallas for imaginary $250 giving 14 points at home Saturday against Atlanta. This is a big number, but a legitimate one. At 13 1/2 the betting world would have beaten a path to the Cowboys' locker room door.

Atlanta has absolutely no chance to run the ball on Dallas.This means the Cowboys will be teeing off when Steve Bartkowski drops back to pass. He will be dropped, repeatedly. Conversely, Atlanta's gambling defense stands to be exploited by Roger Staubach, who has the weapons needed to make the Falcons temper their occasional nine-on-the-line approach to defense.

The fourth playoff game concerns Minnesota at Los Angeles. The Rams should win, but the spread is 7 1/2 points and L.A. Coach Ray Malavasi is as conservative as they come. He will sit on 10 points, and the Vikings' Fran Tarkenton is likely to scamper for three or seven points in the closing minutes. Still, I will risk an imaginary $100 on the Rams.

I asked Bob Martin, the head linesman in Las Vegas, to project lines on the conference championship games, should the favorites win this weekend. Martin makes Pittsburgh 4 at home over New England and Dallas 3 1/2 at Los Angeles, with Dallas 4 over Pittsburgh in the Super Bowl should both get there.