Don't use scriptlets in the struts tags, don't use it at all in JSP, if you can't you may try
<s:set var="loginPassword"><%=OpeCommon.LOGIN_PASSWORD %></s:set>
<s:param name="%{#loginPassword}"><%=OpeCommon.encriptPassword(OpeCommon.KEY_USER_ID, OpeCommon.KEY_USER_PASSCODE)%></s:param>
Note, that constants could be accessed through the OGNL expression, and that expression is allowed in the struts tags. But, it applies to the OGNL syntax and you should supply FQCN to the static content. Constants don't require enabling static access in the configuration, but calling a static method is required. If you allow static access then everything could be done via OGNL expressions, if not only constants could be accessed like
<s:param name="%{@com.package.OpeCommon@LOGIN_PASSWORD}">
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