I've been following Robin Olsen for a long time and not surprised that he has done quite well for you so far. Before Roma bought him after the World Cup, he made a bunch of great games prior to that with the Swedish national team against top nations as Italy, France and Netherlands. Remember that he was brought to Roma to replace Alisson and if you get an opportunity like that, then you can't be too bad. I think he did some great games in the first half of the season for them and when they lost it was usually not his fault, mainly due to a inferior central midfield compared to the previous season. They sold both Nainggolan and Strootman the same summer as Olsen arrived. The central defender Fazio didn't look solid either. They changed manager and Olsen became a bit of a scapegoat and was seen as fourth keeper after the season. He had one of the best goalkeeping coaches in Savorani, same guy that transformed first Szczesny and later Alisson. Many teams in the middle and bottom of Italian, French, German leagues were interested to sign him on loan but they didn't want to pay his full salary. In the last days of the transfer window Cagliari came to an agreement with Roma. Their first keeper and Italy's number 2 Cragno had got a severe injury and was out for half a year. Cagliari as a team overperformed that first half of the season 2019-2020 and Olsen was ranked as one of the best goalies in Serie A in several criterias. Olsen got a 4 match suspension before the winter break and that was the beginning of Cagliari's decline. He played some games before Cragno was fully recovered but the team never regained the form. Cragno was too valuable to not be included in the starting eleven when he was ready and then Olsen was benched for most of the remaining games. Cragno is younger and a great keeper in general but the team played better with Olsen that season in my opinion. I assume Ancelotti is fully aware of Olsen's capabilities as being Italian and also a former player for Roma.
My conclusion is that he could be a bargain considering his relatively low market value. He has improved his kicking slightly in Italy and worked a lot with Savorani on the low shoots. He's not Alisson or Oblak but how many in the world are as good as them.