This example uses OpenLayers v10.9.0. The latest is v.
Edit

#### Custom Circle Render

circle 4 feature 12 vector 86 render 4 custom 6

#####

Example of a custom circle render.

This example demonstrates the use of 'ol/style/Style' render option function to render circle feature.

##### main.js

    import Feature from 'ol/Feature.js';
    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import Circle from 'ol/geom/Circle.js';
    import TileLayer from 'ol/layer/Tile.js';
    import VectorLayer from 'ol/layer/Vector.js';
    import OSM from 'ol/source/OSM.js';
    import VectorSource from 'ol/source/Vector.js';
    import Style from 'ol/style/Style.js';

    const circleFeature = new Feature({
      geometry: new Circle([12127398.797692968, 4063894.123105166], 50),
    });
    circleFeature.setStyle(
      new Style({
        renderer(coordinates, state) {
          const [[x, y], [x1, y1]] = coordinates;
          const ctx = state.context;
          const dx = x1 - x;
          const dy = y1 - y;
          const radius = Math.sqrt(dx * dx + dy * dy);

          const innerRadius = 0;
          const outerRadius = radius * 1.4;

          const gradient = ctx.createRadialGradient(
            x,
            y,
            innerRadius,
            x,
            y,
            outerRadius,
          );
          gradient.addColorStop(0, 'rgba(255,0,0,0)');
          gradient.addColorStop(0.6, 'rgba(255,0,0,0.2)');
          gradient.addColorStop(1, 'rgba(255,0,0,0.8)');
          ctx.beginPath();
          ctx.arc(x, y, radius, 0, 2 * Math.PI, true);
          ctx.fillStyle = gradient;
          ctx.fill();

          ctx.arc(x, y, radius, 0, 2 * Math.PI, true);
          ctx.strokeStyle = 'rgba(255,0,0,1)';
          ctx.stroke();
        },
      }),
    );

    new Map({
      layers: [
        new TileLayer({
          source: new OSM(),
          visible: true,
        }),
        new VectorLayer({
          source: new VectorSource({
            features: [circleFeature],
          }),
        }),
      ],
      target: 'map',
      view: new View({
        center: [12127398.797692968, 4063894.123105166],
        zoom: 19,
      }),
    });

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>Custom Circle Render</title>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <style>
          .map {
            width: 100%;
            height: 400px;
          }
        </style>
      </head>
      <body>
        <div id="map" class="map"></div>

        <script type="module" src="main.js"></script>
      </body>
    </html>

##### package.json

    {
      "name": "custom-circle-render",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }