The ceremony at the Royal Chapel (Slottskyrkan) got under way at around lunchtime on Friday.
Three-month-old Oscar's older sister Princess Estelle was at the church together with their cousins Princess Leonore and Prince Nicolas, joined by their parents, grandparents the King and Queen, and a host of other dignitaries.
Estelle with her dad Prince Daniel and mum Crown Princess Victoria. Photo: Anders Wiklund/TT
The service was conducted by archbishop Antje Jackelén, and assisted by bishop Johan Dalman and chaplain Michael Bjerkhagen.
Oscar stayed mostly calm during the ceremony, even falling asleep in the arms of his mother, Crown Princess Victoria. Estelle and Leonore meanwhile played on the floor next to the christening font.
Estelle, Oscar, Victoria and King Carl XVI Gustaf. Photo: Jonas Ekströmer/TT
Swedish public broadcaster SVT aired it live for domestic viewers, good news for those hoping to catch a glimpse of the traditional 21-shot salute that was set to follow the ceremony. Stockholm was drenched in rain on Friday morning, and the showers were set to continue throughout the day.
Oscar wore the same baptism gown donned by all members of the royal family before him since 1906, including Estelle, Leonore and Nicolas.
Princess Madeleine's husband Chris O'Neill and Oscar's cousin Nicolas. Photo: Maja Suslin/TT
After the ceremony there was a reception and lunch for the invited guests. Along with Swedish royalty, dignitaries and politicians, members of both the Danish and Norwegian royal families are set to attend.
Invited too were obstetrics professor Lennart Nordström and midwife Cecilia Lind, who brought Oscar into the world back in March.
Absent from the guest list however was the latest addition to the Swedish Royal Family, Prince Alexander. The tiny son of Prince Carl Philip and Princess Sofia was only born on April 19th, and appears to have the day off.