^^^^ It depends on the strap-on dildo, how it attaches to the harness, and where the dildo pushes against the woman's body.
Most of the dildos go through a ring on the harness and are held in place by being passed through the ring from the woman's side poking forward. Others are held onto the harness by snaps and the ring. I like the harnesses that have a ring held on by snaps because you can change the ring for different sized (especially BIG) dildos. The ring is what limits the diameter of the dildo.
A few strap-on dildos have a second penis that enters the vagina while the first one enters the other person, like the Feeldoe. Those have offset penises, the one going into the vagina being lower down than the one entering the pother person.
Now, if the strap-on harness, or in the case of the double dildo strap-on, has the primary dildo at the height of the clitoris, you can cum while using a strap-on. OK, now to answer your question, can a lady have an orgasm while using a strap-on, my answer is that it's possible but not common. The strap-on pounding on the clit is sometimes too hard to have an orgasm or not well directed, and I, like many women, like circular stroking over the clitoris and labia to have an orgasm, and the strap-on doesn't do that. I have had a few orgasms that way, but not enough to be an appreciable number in a lifetime of orgasms. I certainly would not go out and buy a strap-on as another means of achieving an orgasm on a regular basis.