diff --git a/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.scss b/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.scss
new file mode 100644
index 0000000000000..6b55a43201d16
--- /dev/null
+++ b/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.scss
@@ -0,0 +1,3 @@
+.actOf__aggFieldContainer {
+  width: $euiSize * 29;
+}
diff --git a/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.tsx b/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.tsx
index 70aa4be6b7f87..d399f6136690b 100644
--- a/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.tsx
+++ b/x-pack/plugins/triggers_actions_ui/public/common/expression_items/of.tsx
@@ -18,6 +18,7 @@ import {
 import { builtInAggregationTypes } from '../constants';
 import { AggregationType } from '../types';
 import { ClosablePopoverTitle } from './components';
+import './of.scss';
 
 interface OfExpressionProps {
   aggType: string;
@@ -108,7 +109,7 @@ export const OfExpression = ({
           />
         </ClosablePopoverTitle>
         <EuiFlexGroup>
-          <EuiFlexItem grow={false} className="watcherThresholdAlertAggFieldContainer">
+          <EuiFlexItem grow={false} className="actOf__aggFieldContainer">
             <EuiFormRow
               fullWidth
               isInvalid={errors.aggField.length > 0 && aggField !== undefined}