Having started the day eight points adrift of safety, Oviedo began like a team who knew they sorely needed a victory, with Ilyas Chaira forcing Álvaro Valles into a sprawling stop within 15 seconds of kick-off.
Valles was called into action again shortly after when he saved Alberto Reina’s shot on the turn, before Santiago Colombatto whipped a free-kick just wide of the near post.
However, Los Azules’ good work was undone midway through the half when Pablo Fornals’ deflected cross fell kindly for Hernández, and Betis’ top goalscorer in LaLiga tucked home from 15 yards.
Oviedo failed to capitalise on some good positions as they hunted an equaliser, but Betis would have increased their lead were it not for Chaira’s goal-saving sliding interception to deny Abde Ezzalzouli a tap-in at the far post.
The visitors then felt they should have had a penalty when Sofyan Amrabat caught Thiago Fernández with his studs as he slid in on the byline, and their frustrations only grew when Antony crossed for Ezzalzouli to poke home and double Los Verdiblancos’ advantage on the stroke of half time.
Los Azules were on the wrong end of another marginal decision eight minutes after the break when Federico Viñas thought he had halved the deficit with a header, only for VAR to show he was centimetres offside.
Oviedo brought 41-year-old Santi Cazorla off the bench as they looked to fight back, but before he could make an impact, Betis added a third goal, as a counter-attack ended with Hernández flicking Ezzalzouli’s cross in off the post for his 10th LaLiga goal of the season.
Viñas twice headed off target before Valles saved Reina’s shot with his legs, but Betis saw out a comfortable victory to extend their unbeaten H2H run to 14 games (W7, D7).
Pellegrini’s side have not lost at home in the league since December, and they are now six points clear of sixth-place Celta Vigo, which could prove crucial if Spain is awarded a fifth UEFA Champions League qualifying spot.
In contrast, Oviedo look set to make an immediate return to the second tier with just four games of the LaLiga season remaining.