Sounds like you need the
URL Rewrite[
^] module.
<system.webServer>
<rewrite>
<rules>
<rule name="Site Skin">
<match url="^(OEM)/([^?]*)(.*)$" />
<action type="Rewrite" url="{R:2}?_skin={R:1}" appendQueryString="true" />
</rule>
</rules>
</rewrite>
</system.webServer>
This will redirect
oem/anyPage.aspx?query
to
anyPage.aspx?_skin=oem&query
; you can then use the
_skin
query-string variable to decide which skin to use.
Extending it to other folders is as simple as adding another value to the
<match>
element:
<match url="^(OEM|OEM2|Another)/([^?]*)(.*)$" />