The OpenMP memory model has been evolving. Up to and including OpenMP 3.1, the model is based on "flush" operations. These are not really comparable to anything in the C++ memory model. Loosely speaking a flush roughly corresponds to atomic_thread_fence(x), where x is memory_order_seq_cst. But there's a gotcha where it's not that if the "intersection of the two flush-sets of the two flushes is empty".
OpenMP 4.0 adds the seq_cst clause, and answers the question directly:
Note – As with other implicit flush regions, Section 1.4.4 on page 20 reduces the ordering that must be enforced. The intent is that, when the analogous operation exists in C++11 or C11, a sequentially consistent atomic construct has the same semantics as a memory_order_seq_cst atomic operation in C++11/C11. Similarly, a non-sequentially consistent atomic construct has the same semantics as a memory_order_relaxed atomic operation in C++11/C11.
For more details, download the latest version of the spec (4.0 at this time) and read sections 1.4.4 "OpenMP Memory Consistency" and 2.12.6 "atomic Construct".