Opheus and Dermott's parentage... SPOILERS

My reading of the situation was that he is a reluctant and terrible liar. He came in, was surprised to find Dermott, said what he said, and couldn't thereafter weasel his way out of the hole he had dug.

I guess the issue is why he would have been surprised in the first place, knowing what he knows from "Everybody Comes to Hank's." My guesses, in no particular order:

1. He may have memory wiped himself (as Hank and Al chose to) and we just didn't see it. Given his preference for magic over science, he would have likely done it to himself somehow instead of using SPHINX's machine. We also know he knows how to, since he's done it to Triana in the past.
2. He may have been so surprised that he just blurted it out without thinking. This seems less likely, as he's usually pretty composed; he was pretty calm when facing that Cthulhu-looking monster in "The Better Man", which was a far more serious situation than this (though as a counterpoint, he did get freaked out pretty easily by the images he saw in "What Goes Down Must Come Up").
3. He may have done it intentionally. I don't know how likely this is; Dr. Orpheus seems to be more of an observer and moral guide (i.e., telling people what they should do, not doing it for them), especially when it comes to his landlord Dr. Venture's business. It seems incongruous that he would tell Dermott that he's a Venture intentionally, but not do anything to help the boys figure out that they're clones, which he was shown to be pretty upset about in "Powerless in the Face of Death."
4. He may have underestimated some combination of his own powers and Dermott's power of perception. He could have been surprised that he would be able to sense Dermott, since Dermott likely never tried to reach out to him before. Similarly, the statement "I thought I sensed one of the Venture boys" could have just as easily have been shrugged off (since Dermott probably has no idea how any of Dr. Orpheus' magic works) as delved into by Dermott; Dr. Orpheus could have miscalculated the extent to which Dermott would harp on it.

Of these, I personally like 1 and 4 the best, though it troubles me that we didn't see the mind wipe that makes 1 a possibility.

To close, as always when posting opinions, I must emphasize have no special insights into the show or its creators, and the answer could be just as easily be something completely different as one of my suppositions.