Testimony begins Tuesday in the trial of an Oakdale man charged in a Nassau police officer’s 2012 death — a case experts said could be more complex than usual for jurors since it wasn’t the defendant’s car that hit the victim.

Prosecutors have alleged James Ryan was driving drunk when he caused the Oct. 18, 2012, death of veteran cop Joseph Olivieri Jr. on the Long Island Expressway near exit 35 in North Hills.

They contend Ryan, now 28, is responsible because Olivieri was “suddenly and violently killed” responding to two other crashes Ryan caused.

“There’s a little bit more here for a jury to digest than you would have in a typical vehicular homicide case,” said Joseph Petrosino, a retired prosecutor who once led the vehicular crimes section at the Brooklyn district attorney’s office.

The defendant, the son of a retired Port Authority police official, has pleaded not guilty to charges including aggravated vehicular homicide, vehicular manslaughter, manslaughter and criminally negligent homicide.

The defense is expected to argue Olivieri’s death happened because of the reckless actions of a Cadillac Escalade driver who struck and killed the officer at the scene in what was a separate incident.

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“This jury is going to have their hands full,” said Garden City defense lawyer Steven Epstein, who specializes in vehicular cases.

In 2013, a Nassau judge dismissed several criminal counts against Ryan, including the top count, finding Olivieri’s death “solely attributable” to the Cadillac driver. The judge decided the five to 10 minute gap from the first crash to the fatal wreck wasn’t a continuing chain of events.

But an appellate court disagreed, saying in 2015 there was legally sufficient proof before the grand jury that Ryan’s actions “caused” the officer’s death. That court said it was “reasonably foreseeable” Ryan’s conduct would cause collisions and police would respond and face potentially lethal traffic.

Attorney William Keahon of Hauppauge, who has handled high-profile vehicular cases, said the appellate court’s decision to let the charges stand “in no way compares to legal sufficiency before a jury as far as proof beyond a reasonable doubt.”

The Cadillac driver got immunity after testifying before the grand jury. Prosecutors decided that motorist’s actions weren’t criminal.

Joseph McCormack, vehicular crimes bureau chief of the Bronx district attorney’s office, said the fact that another motorist hit the officer “will be a big part of the case from both sides” during a trial in which jurors will be called upon to understand the legal concepts of causation and foreseeability.

“It’s always critical for jurors to understand it’s not about, “Did he intend the officer to die?’ ” said McCormack, who teaches at St. John’s University School of Law. “It’s ‘Did he set up a situation where the officer has to pay for the defendant’s choices?’ ”

Keahon, who said he expects the jury “is going to be very, very careful,” was among experts who agreed jurors are likely to ask for the law to be repeatedly read back to them as they deliberate.

Olivieri was 43 when he died and an LIE overpass has been renamed in his honor.