The Forge OreDictionary is like a huge Lexicon in the way that it features many different entries and each entry is described by fitting Items.
When talking about ore Dictionarys, people mostly refer to IOreDictEntries not the whole ore Dictionary.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.oredict.IOreDict;

You can retrieve the OreDictionary using the oreDict global keyword.

So what can be done with the oreDict?

You can use either of the three ways below to refer to an OreDictEntry, no matter if it already exists or not.
If the oreDictEntry does not yet exist, it will be created.

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oreDict.ingotIron;
oreDict.get("ingotIron");
oreDict["ingotIron"];

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//the entries member is for retrieving all registered oreDictionary entries!
val allEntries = oreDict.entries;

for oreDictEntry in allEntries{
    print(oreDictEntry.name);
}

//Alternatively, you can just iterate through oreDict
for entry in oreDict {
    print(entry.name);
}

You can use the in or has operator to check if an IOreDictEntry already exists:

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if (oreDict in "ingotIron") {
    print("ingotIron exists!");
}

if (oreDict has "ingotIron") {
    print("ingotIron exists!");
}


//Alternatively use the contains function:
if(oreDict.contains "ingotIron") {
    print("ingotIron exists!"); 
}