How to draw a triangle by turning it 90 ° anticlockwise around its outer point?

How to draw a triangle by turning it 90 ° anticlockwise around its outer point?

Triangle ABC, a point o on the outside
Connect OA, draw OA vertical line through O, and cut OA '= OA counterclockwise
Do OB and OC in the same way`
The new triangle is a ` B ` C`