Pre-heat oven to 350F.
Combine rhubarb, strawberry and lemon juice in saucepan. Cook on medium heat for 10 minutes (until tender). Add sugar and cornstarch. Cook 2 minutes (until boil and thickened). Remove from heat.
Combine and mix flour, oartmeal, sugar, butter, baking soda, and salt in bowl (until resembles small crumbs). Reserve 1 1/2 cups crumb mixture. Press rest onto bottom of greased 13x9 baking pan. Spread filling over crust. Sprinkle with reserved crumb mixture. Bake for 30-35 minutes (until golden brown). Remove from oven and let cool.
Combine powered sugar and milk in bowl, drizzle over cooled bars. Cut into bars.