Jon Lancaster had started the season with Ocean, but was replaced by Hartley for both rounds in Bahrain after struggling to find the budget to complete the season.
Guerin currently lies eighth in the Auto GP standings after eight races driving for Super Nova, but he is aware that he will have a lot to learn in Spain.
"I know Barcelona well, but the task will not be easy for me because I have to learn the car, which is totally new for me, in just a half hour practice," he said.
"But I am relaxed, I want to get the most out of this experience."
Ocean team bosses Tiago Monteiro and Jose Guedes agreed that the young Brazilian driver will have a lot to learn in the time available.
"We are very content to have Victor come to Ocean. He is a very young driver but he has immense talent," they said.
"It will be a very big challenge for him but we are here to help him. The objective is for him to get a good experience by the weekend."
The other driver change on the grid for this weekend comes at Addax, where Josef Kral returns to the driver line-up after being replaced by Dani Clos for the two Bahrain rounds last month.