Make a text field required if pick list value equals specific value

I am trying to make a custom object 6 field required based on the value entered in another field

My picklist field name is "SS Category 2" (IntexedPick1) and if the values are Other specify, Other - Specify or Other VAR specify then I want the text field "Other Specified" to be required.

I've placed the following on the "SS Category 2" field validation section.

[<IndexedPick1>] =  "Other Specify" OR [<IndexedPick1>] = "Other -  Specify" OR [<IndexedPick1>]= "Other VAR specify" AND [<stOther_specified_ITAG>] IS NOT NULL

It works, but it also works no matter what choice I make in "SS Category 2".  The field is required no matter what value I select.