O.J. Howard most likely will be gone when Denver picks at #20. The only way Denver could get him is to trade up; way up. He is not known for his blocking from the tight spot on LOS, but is a very good receiver who has a lot of upside as one. As a prospective NFL TE, one would hope his blocking has upside, too.
Christian McCaffrey may very well be available to Denver at #20. He is a different kind of offensive skill guy, so comparing him to Howard is apples/oranges.
"Stultum est timere quod vitare non potes." ~ Publilius Syrus