OSPF - ECMP across two different areas

OSPF will perform Equal-cost Multi-path routing (ECMP) or load balancing if the cost of the multiple paths are exactly the same. However, OSPF will not perform this load balancing if the destinations can be reached via multiple areas.

For example, an ABR that has two equal cost paths to a destination of via Area 0 and Area 1 will not perform ECMP. There is no definitive explanation for this in Cisco documentation, however, RFC2328 that describes OSPFv2 indicates the following:

There is no requirement that a router running OSPF keep track of all possible equal-cost routes to a destination. An implementation may choose to keep only a fixed number of routes to any given destination. This does not affect any of the algorithms presented in this specification.

So OSPF does not obligate a device to always perform ECMP, and this may be a vendor-specific behavior or even an IOS-specific behavior.