The Harambee Stars striker scored three goals in a span of 22 minutes to aid Girona to ninth place on the league table following a massive win over bottom-placed Las Palmas in a one side match.
Olunga opened his goal account in the 57th minute to double Girona’s advantage after Christian Stuani had given them the lead from the spot in the 25th minute.
The former Gor Mahia, Tusker and Thika United striker then doubled his effort in the 70th minute before completing his hat-trick nine minutes later.
Olunga's striking partner Borja Garcia and Portu contributed a goal each in the 64th and 74th minutes respectively. Girona have now moved joint 9th with Getafe on 26 points, though the latter have a superior goal advantage.
Leaders Barcelona are top of the table with 48 points, nine points ahead of second-placed Atletico Madrid while defending champions, Real Madrid are fourth behind Sevilla on 32 points.
Olunga also became the first Kenyan to have scored in La Liga and the first Girona player to score a hat-trick in the top tier competition.
The three goals took Olunga's total tally for Girona to five having scored a brace on his debut in a friendly against Ripoll last September.