I just looked through their premier league records to compare.
Unless I hit a button wrong on the calculator:
-Niasse played 3337 Premier League minutes (with a few less than half his appearances coming from the bench) and scored 12 goals.
That works out to a goal for about every 278 minutes played.
-Rondon has played 10917 PL minutes (almost entirely played as a starter) and scored 35 goals.
That works out to a goal for about every 312 minutes played.
A person could reasonably analyze a lot more to compare their time in England, but in terms of just the raw "how many goals did you score in the time you were given?" Oumar absolutely comes out ahead.
If you compare their time in Russia, Rondon was once every 126 minutes and Oumar once every 146 minutes.
The only particularly impressive time in Rondon's career was when he was 21 and 22, scoring goals for Malaga. But I'd think Everton fans would typically disregard goals scored in the distant past for Malaga, especially considering Sandro scored more often for them than Rondon did.