The Opaque type is provided solely for backward-compatibility, and shall not be used for newly-defined object types.
The Opaque type supports the capability to pass arbitrary ASN.1 syntax. A value is encoded using the ASN.1 Basic Encoding Rules  into a string of octets. This, in turn, is encoded as an OCTET STRING, in effect "double-wrapping" the original ASN.1 value.
Note that a conforming implementation need only be able to accept and recognize opaquely-encoded data. It need not be able to unwrap the data and then interpret its contents.
A requirement on "standard" MIB modules is that no object may have a SYNTAX clause value of Opaque.