<aura:attribute name="selectedCriteria" type="c.MyApexClass1" />

var selectedCriteria = component.get("v.selectedCriteria");
var action = component.get("c.getOpportunities");
action.setParams({"searchCriteria" : JSON.stringify(selectedCriteria)});
action.setCallback(this, function(response) {
var state = response.getState();
if (state === "SUCCESS") {

}

@auraEnabled
public static List<MyApexClass2> getOpportunities(String searchCriteria)
{
MyApexClass1 criteria=
(MyApexClass1)JSON.deserialize(searchCriteria,MyApexClass1.class);
}