Read the release notes for every intermediate version very carefully.
Do the migrations in steps - i.e. plan the move from Oracle 10g to 11g separately from the upgrade
Make sure your upgrade path for Confluence is 3.0.2 -> 3.5 -> 4.3.7 (you must got through 3.5)
You can, of course, do the database change at the same time as an upgrade, but it's better to change one thing at a time. If you do both at the same time, and something goes wrong, it won't be so easy to roll back of identify the error. Even if you plan to do the whole lot in one single run, it's still worth ring-fencing it into three distinct parts, at the very least for the testing before you upgrade live.
Just in addition to the previous answer, to build a test environment to perform the upgrade in a first moment before apply in production environment, you can follow these steps:
I hope this helps.
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