Checked the routing for YSEFLEX ARN-BRU on expertflyer.
This is what I got, seems like you have to choose between OSL or CPH or buy two separate tickets.
FARE BASIS CODE
YSEFLEX
QTE F/B BK FARE EFF EXP TKT AP MIN/MAX RTG
1 - YSEFLEX Y X 4060 - - - - -/ - EH01
*** NO APPLICABLE YY NLX TYPE FARES FOUND STO-BRU **
EH01 /WITHIN THE EASTERN HEMISPHERE/
STO-CPH/OSL-BRU