Most operating expenses are tax deductible but there are certain operating expenses that you can not use to reduce your tax bill.
As long as the expenses are incurred wholly and exclusively for the business operations, they should be tax deductible.
To be more specific, there are certain expenses that the tax authorities might not accept as legitimate operating expenses for tax purposes. For example, client entertainment expenses are not usually allowed to be used as an expense for tax purposes.
The best thing to do is actually have a look at the IRS (or HMRC for the UK) website and find examples of expenses that are accepted because things tend to change (more often than they should).