I'm fairly certain the number of webshops running on oracle can be counted on one hand. It's like killing a fly with a missile; it doesn't make any sense.
Oracle is mostly found in "enterprise" environments, i.e., large corporations.
And I agree with others that MySQL should well be able to handle this volume. 100,000 orders a year ~= 11,4 orders an hour. A database has to be really poor to not be able to handle that. Hack, even SQLite should be able to handle that if you ask me.
If you want an alternative you can also look at PostgreSQL, which in my opinion is faster and in my experience more stable than MySQL. Plus it supports master-master replication (MySQL only supports master-slave), which is good for scaling.
Oh, and twitter also runs on MySQL. I wonder why people say it doesn't scale, it clearly does.