mock.m

@interface Calculator : NSObject
- (NSNumber *)methodA;
- (NSNumber *)methodB;
@end

@implementation Calculator
- (NSNumber *)methodA {
    return [self methodB];
}
- (NSNumber *)methodB {
    return @0;
}
@end

Calculator *originalObject = [[Calculator alloc] init];
Calculator *partialMock = OCMPartialMock(originalObject);
[OCMStub([partialMock methodB]) andReturn:@42];
XCTAssertEqualObjects([partialMock methodA], @42, @"Expecting 42");
XCTAssertEqualObjects([originalObject methodA], @0, @"Expecting 0"); // Fails here, returns 42