And although he never talked about the war with his granddaughter, stories, photos and memorabilia have passed down to her.
Mr Oppenheim joined up at the outbreak of the war in 1914. He was already a territorial soldier but his job in the Post Office gave him skills that the army could not overlook.
One of her treasured photos was taken at the expeditionary force's camp at Zeitoun in Egypt. Faded and barely decipherable writing on the back showed it was the first mail from Zeitoun Camp, 1914. Another photo showed men sorting mail in the base post office in Cairo.
Mr Oppenheim, who picked up his nickname from being a solemn serious minded child, landed at Anzac Cove in May 1915, a couple of weeks after the first troops went ashore.
He sorted mail with a small group on the floor of the dugout at a time, when the hillside still came under regular shelling and sniper fire.
Mr Oppenheim seldom talked about his experiences and never to his granddaughter, but she has inherited stories passed on to her grandmother. One story, verified by a newspaper report, happened when he managed to find a rare treat of a dozen eggs to break their monotonous diet of bully beef and biscuits.
He was returning with the eggs along Anzac Cove beach when a Turkish artillery piece called Beachie Bill opened up. Mr Oppenheim threw himself flat on the ground while somehow managing to hold the eggs up - the eggs survived.
Almost all his stories that found their way home downplayed the horrors, because he did not want to spread alarm and despondency among loved ones at home.
Ms Smith described his stories as bracing, with a laborious sense of humour, like his reference to sleeping on a "down bed" - his bed was down on the ground.
Another story was that his voice meant he was given the job of reading dispatches aloud every morning, presumably to a meeting of officers. And working near the beach meant the water was too much of a temptation for a swim, even though nowhere was safe from snipers.
None of his stories mentioned the parcels and letters that never reached their recipients because they had been killed. A death in the family happened when Mr Oppenheim's cousin Langley Manning was shot coming ashore at Gallipoli.
His experiences at Gallipoli had a grim aftermath after Mr Oppenheim had returned home and resumed his love of gardening. He hated rhododendrons because of the terrible word association with Gallipoli's Rhododendron Ridge, a spur which ran up from the beach to Chunuk Bair, along which hundreds of New Zealand soldiers were killed or wounded in the push to reach the peak.
The evacuation from Gallipoli saw Ms Smith's grandfather move with the rest of the Anzac forces to the Western Front. There his dedicated approach to the job saw him promoted to the rank of deputy assistant director of army postal services, with service in France and England. She guessed he never got near the trenches after that.
Mr Oppenheim lived to the age of 75, with continuing stomach problems a legacy of his harsh life on Gallipoli.