Donczew, a 16-year-old attacking midfielder, joins as part of the Premier League champions' first-year scholarship intake after Cardiff's efforts to keep him fell short. The Bluebirds say they offered terms in an attempt to retain the youngster, but confirmed he "opted to pursue a move away from the Welsh capital."
Donczew made history at his boyhood club last October when he became Cardiff's youngest ever senior player - a record since broken by Noah Anyadike - coming on as a substitute in an EFL Trophy tie against Newport County aged just 15 years and 234 days. He added a second first-team appearance in December, again from the bench, in a defeat by Wimbledon.
Eligible for both Wales, whom he has represented at youth level, and Poland, Donczew has signed a pre-contract agreement that Arsenal say will "lead to him signing professional terms" once he is old enough.
Ogunneye's arrival, meanwhile, completes a full-circle moment for the young defender, who grew up in Hackney before joining Manchester United's academy.
"It feels great to be here, it's such a big club," he said after signing. "I've got family who live in Hackney, so it's literally a full-circle moment. It's fantastic to have my family here." Beyond the personal connection, Ogunneye pointed to Arsenal's recent record of promoting academy players as the deciding factor in his decision.
"When I heard Arsenal were interested in me, I said to my agents it's a no-brainer. Seeing all the young players that are coming through as well, so I just wanted to come straight away. I just wanted to be part of that group, and win games together. It's good playing with good players, good defenders; it's only going to better you."
Ogunneye already has senior experience under his belt, having spent the first half of last season on loan at Newport County, where he scored and helped the club avoid relegation.
"It was great, my first season in men's football, learning about the physicality, the recovery side, it was a great experience," he said. "I remember when I scored, it was like a moment of madness. I didn't really know what to do. The stadium erupted; that's the first time it's properly happened for me, being in a big stadium where the three points mean something."
Looking ahead, he added: "I want to get better every day and we'll see what happens from there."