No particular sensation at all, unless you have some medical condition that causes you to need it, or are in a situation where there isn't any air. Then it's the difference between breathing normally - or near normally - and and not breathing at all.

FAA regulations require (or did the last time I checked a few years ago) that in commercial airlines carrying passengers for hire at least one member of the cockpit crew must be breathing pure oxygen anytime the plane is above a certain altitude. There thus is frequent use, and the ones who are the designated breathers are no more (or less) crazy from it than the ones who breathe normally and rely on the pressurization. (That is so far as I've been able to tell, at least. You need to have enough exposure to aircrews to different the usual deviant personality traits from anything that might be the result of any variant exposure.)

Breathing NO OXYGEN also produces, according to those who've done it, no "felt effects," so long as there is "another gas" to go in and out to "simulate breathing." Those wishing certification as "test pilots for hire" generally are required to pass "hypobaric chamber" training, and the usual procedure is to put a group in the chamber, run it up to "altitude," and have one of them remove the oxygen mask and tell a story. When the story-teller passes out, someone slaps a mask back on him/her. The "victim" usually denies having been unconcious, and usually continues the story, continuing in mid syllable of the word at which conciousness was lost. The refusal to accept that anything happened is often "adamant" so the schools nearly always tape the lesson for playback - mainly for the benefit of the victim.

Any physiological difference (to make you feel different) would depend on your bloodstream being able to absorb more oxygen in a pure oxygen environment. Since most people can maintain 95 to 98% saturation¹ of the hemoglobin they have even in "normal air" there's little room for much more to be absorbed to produce any "feeling."

¹ The "normal range" for pO2 saturation of the blood goes somewhat below 95%, but most people can achieve that level just by taking a few deep breaths - in the absence of a physiological malfunction or a badly contaminated atmosphere.

(Not a medical opinion, but based on lots of observation and some personal testing.)

John