[You neglected to include your Quicken Edition (example: Deluxe) and your Quicken Build (example: 16.1.16.2). That information can be important to diagnosing your problem: you should always include that information when you initiate a discussion here.]
Here's a tip: an opening balance transaction is not "spending" (nor "income").
Also, an opening balance transaction is a "transfer".
Quicken - logically - excludes ALL transfers from Spending by Category reports ... by default. And Quicken excludes all opening balance transactions from virtually all Quicken Spending reports ... by default.
Quicken does NOT "subtract" opening balance transaction from reports; but Quicken sometimes does not include opening balance transactions in reports ... by default. See above.
You can elect to include opening balance transactions in most reports, using the Advanced tab in the report Customize dialog. In the "Transfers" dropdown, select "Include All".
A Quicken account "opening balance" transaction represents the balance of all activity that occurred prior to the entry of transactions in the account.
A Quicken account "opening balance" transaction is recorded in Quicken as a "transfer back into the same account". That means its Category is the name of the account where the opening balance is recorded ... in square brackets. If what you are calling an "opening balance" transaction is not recorded that way; it is not, what we would refer to as, an account "opening balance" transaction.
How did your "opening balance" transaction get into your Quicken account; and how, exactly, is it recorded?
If the date of your Quicken account opening balance transaction is prior to the beginning date of your report, that transaction should not appear in the report. Amounts received (income), or spent (expenses), prior to the starting date of a report ... should not appear in the report.
I have no idea why you are suggesting that excluding such a transaction from a report would have anything to do with causing you to be out of balance with your real-world account. The date-range of Quicken transactions you choose to include in a Quicken report has nothing to do with keeping your Quicken account balance in sync with your real-world account balance.
[And - as I noted previously - if you want the Quicken opening balance transaction to appear in the report (despite the fact that it does not seem, based on your problem description, that it should be there), you can control that with the report Customize > Advanced dialog for Transfers, or the Customize > Categories dialog by selecting/deselecting the appropriate transfers into/out of specific accounts.]
"I guess I could change that date to 12/31/17, so it wouldn't appear in a 2018 spending report."
Yes, you could do that - and sometimes that may be the best way.
In general, you can usually handle those transactions differently for reports.
Quicken calls transactions that transfer funds back into the same account (typically, opening balance transactions and adjustment transactions), "self-transfers" (that's a new name for them, starting in Q2018, I believe).
You can exclude self-transfers from most reports by going to the Customize > Advanced tab and selecting "Exclude self-transfers" from the "Transfers" dropdown. [If you have a version earlier than Q2018, try "Exclude internal" in the same dropdown.]
