Romelu Lukaku opened the scoring when he slid under Heurelho Gomes for his first goal in five games.
But Okaka’s fine back-heel levelled the game and he headed home his second after Sebastian Prodl netted around the hour mark.
Lukaku scored his second of the game late on to make it a nervous finish, but the hosts saw out the closing minutes to move up to seventh in the Premier League.
Lukaku opens scoring at Vicarage Road
Walter Mazzarri’s side came into the game having lost three of their final four games, and both sides made two changes apiece.
The visitors started the better and went ahead on 17 minutes when they took full advantage of Lukaku winning the ball deep inside Watford territory.
Gareth Barry’s inch-perfect ball over the back line allowed Lukaku to break through and slot the ball under the approaching ‘keeper.
The Hornets responded well after going a goal behind and had chances to level the game through Okaka and Adlene Guedioua.
Okaka scores first goal for Hornets
Watford managed to find an equaliser nine minutes before the break as Okaka notched up his first goal for the Hornets since signing in the summer.
The Italian forward produced sublime finish with a spinning back-heel to help Nordin Amrabat’s cross past Maarten Stekelenburg and into the net.
Seamus Coleman had the ball in the back of the net, only for it to be rules out for offside, but the hosts took the lead just before the hour mark.
Hosts make set-pieces count
Prodl nodded over a good chance moments before the centre-half made amends and headed home Jose Holebas’ free-kick from close range.
Everton were struggling to deal with Watford’s set-pieces and the hosts took full advantage of that factor once more five minutes later.
Stekelenburg produced a brilliant low save to deny Miguel Britos, however, from the resulting corner, Okaka glanced Holebas’ delivery past the Dutch international for his second of the game.
Lukaku nets late, but Watford hold on
The Hornets looked like they they were going to see out the game comfortably, but Everton managed to pull one back four minutes from time to make it a nervous finish.
Aaron Lennon’s dinked ball into the box was headed past Gomes powerfully by Lukaku and there was a sense of urgency about the Toffees from there.
Lukaku struck from range straight at the Brazilian ‘keeper before Ross Barkley curled a free-kick over the wall, but Gomes had it covered.
Watford closed out the closing minutes and bounced back after back-to-back defeats with a brilliant and spirited display in front of their own fans.