The simple way to add an OEIS search to the search bar in most browsers is to visit an OEIS page (such as this one), and then the search bar pulldown will offer to add the OEIS to the list of search engines. This works with Firefox (1.5 or later), Google Chrome and Internet Explorer (7 or later).
For Google Chrome, to use OEIS search, right-click on the "omnibox" and select "Edit Search Engines" (or wrench, Options, Manage search engines...). An extension such as Simple Search may make this easier to use.
Alternatives, and more details:
To add an OEIS search bar to Firefox (1.5 or later), Internet
Explorer (7 or later), Google Chrome or Mozilla Seamonkey (and perhaps Camino)
To add an OEIS search bar to Opera (version 9 or later), go to the
main lookup page, right
click in the text box, and select "Create search...". You can also
add the OEIS to the search bar using Tools -> Preferences -> Search.
For Safari, you need to use an extension to have customizable search. OpenSearch is a simple extension that adds OpenSearch capability to Safari. If it is installed, you can add OEIS search to its search set from any OEIS page.
There are also OpenSearch and Sherlock OEIS search plugins available
Thanks to several sequence fans who set up these plugins.