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Depends what you mean by "hide".
AS100's address space "somehow" need to be made known to AS200, so R7 will "know" to forward R1 destined packets toward R1 and the converse is true too, i.e. AS200's address space will "somehow" need to be made known to AS100, so R1 will "know" to forward R7 destined packets toward R7.
AS100 and AS200 could each exchange all their network prefixes, or more likely they will exchange some form of aggregated prefixes; perhaps just one aggregate prefix for all the prefixes in each AS. One of the two AS s might also just advertise the default prefix.
AS200 wouldn't "know" that AS100 is using OSPF nor would it know anything about AS100's interior topology.
