For the vacant WBO heavyweight title in late 2020, assuming the following: Anthony Joshua vacates the belt, as has been widely speculated (it was known prior to Joshua's rematch with Ruiz that whoever emerged victorious would have to dump either the IBF or WBO title, as given the allotted timelines it would not be possible to satisfy both organizations' stated obligations for the next defense. Most expect AJ to elect to keep the IBF strap.) Parker is victorious in his upcoming tuneup on February 29th on the Miguel García vs. Jessie Vargas undercard (opponent TBA). Usyk also doesn't lose a fight in the meantime, and no more attractive offers come along for Usyk before then (ie a Wilder/Fury/Joshua fight, the only significantly more lucrative options than Parker) Usyk is #1 and Parker #2 in the WBO rankings. Parker has a 1" height advantage. Usyk has a 2" reach advantage.
This gonna bum me out, they're my two favorite heavies*; would rather see neither take an L and just have them each separately go after Wilder/Fury & Joshua. (*aside from Kaboom Rivas, but... I've pretty much accepted that he isn't reaching this level at this point)
Given Parker struggled against Hughie "The Technician" Fury I really can't see any way he beats Usyk, other than a 2% chance of a knock out. I see this being a typical frustrating Parker fight where he resorts to chasing Usyk around the ring throwing flailing punches.
A harder fight in terms of styles for Usyk than Chisora, I feel. Probably an Usyk UD but I can't say that with confidence until I've seen Usyk in a proper HW fight.
It means if Usyk grows his hair one inch and parker grows his nails 2 inches........ Wait - what am I saying??