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$ORACLE_HOME/bin/OraOpsn.dll

The n stands for the Oracle version, for example 18 (which would result in OraOps18.dll).
The same OraOpsn.dll is used by the Oracle.DataAccess.dll of both, the .NET Framework 2.0 and 4.

Error message «Unable to load OraOps18.dll»

The error message Unable to load OraOps18.dll was helpfully answered in this Stackoverflow answer.

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