Onana, who was nurtured at La Masia, has been the first choice gloveman at the De Toekomst side for a third successive season and has reportedly been eyed for a possible return at Blaugrana.
Amid reports that Jasper Cillessen - also a former Ajax goalkeeper - is looking to leave Barcelona next summer in search of first-team football, the Cameroonian international has emerged as a favourite to replace the man he superseded at Ajax.
"I think Andre [Onana] is fantastic," Van der Sar told Goal.
"He came to Ajax aged 18 or 19 and the way he developed and the natural attributes that he already had, his speed, his reaction and I think he set himself as a great goalkeeper.
"Also He was in a rush to play first-team football quickly, but I think the level that he composed himself and that first year, playing straight away the Europa League final was a great start.
"Last year was a difficult season but this year again he has proven to be extremely valuable for the team and besides that, he is a great guy.
Onana, 22, has started all 13 of Ajax's Eredivisie and four Uefa Champions League group stage matches keeping 13 clean sheets across both competitions.
Despite the Cameroonian's impressive form, Van der Sar has downplayed the reports linking the Samuel Eto'o Academy graduate with a possible return to Spain.
"I think there are always a lot of rumours about players from Ajax," the 48-year-old old two-time Champions League winner said.
"We develop great players, not only in Amsterdam but hopefully also in Cape Town and there is interest in a lot of our players, but we like to keep them for as long as possible."