Change feature style by attribute. See https://codepen.io/juusechec/pen/OgyzXo

dinamic_style_openlayers.js

var iconFeature = new ol.Feature({
  geometry: new ol.geom.Point([0, 0]),
  name: 'Null Island',
  population: 4000,
  rainfall: 500
});

var iconFeature2 = new ol.Feature({
  geometry: new ol.geom.Point([-1000000, -1000000]),
  name: 'Island 2',
  population: 400,
  rainfall: 500
});

var iconStyle = new ol.style.Style({
  image: new ol.style.Icon(/** @type {olx.style.IconOptions} */ ({
    anchor: [0.5, 46],
    anchorXUnits: 'fraction',
    anchorYUnits: 'pixels',
    src: 'https://openlayers.org/en/v4.1.1/examples/data/icon.png'
  }))
});

var iconStyle2 = new ol.style.Style({
  image: new ol.style.Circle({
    radius: 10,
    fill: null,
    stroke: new ol.style.Stroke({
      color: 'red',
      width: 1
    })
  })
});

var styleFunction = function(feature) { console.log(feature.getGeometry().getType(), iconStyle);
  if(feature.get('population') > 400){
    return iconStyle;
  } else {
    return iconStyle2;
  }  
}

//iconFeature.setStyle(iconStyle);
//iconFeature2.setStyle(iconStyle);

var vectorSource = new ol.source.Vector({
  features: [iconFeature, iconFeature2]
});

var vectorLayer = new ol.layer.Vector({
  source: vectorSource,
  style: styleFunction
});

var rasterLayer = new ol.layer.Tile({
  source: new ol.source.TileJSON({
    url: 'https://api.tiles.mapbox.com/v3/mapbox.geography-class.json?secure',
    crossOrigin: ''
  })
});

var map = new ol.Map({
  layers: [rasterLayer, vectorLayer],
  target: document.getElementById('map'),
  view: new ol.View({
    center: [0, 0],
    zoom: 3
  })
});

var element = document.getElementById('popup');

var popup = new ol.Overlay({
  element: element,
  positioning: 'bottom-center',
  stopEvent: false,
  offset: [0, -50]
});
map.addOverlay(popup);

// display popup on click
map.on('click', function(evt) {
  var feature = map.forEachFeatureAtPixel(evt.pixel,
      function(feature) {
        return feature;
      });
  if (feature) {
    var coordinates = feature.getGeometry().getCoordinates();
    popup.setPosition(coordinates);
    $(element).popover({
      'placement': 'top',
      'html': true,
      'content': feature.get('name')
    });
    $(element).popover('show');
  } else {
    $(element).popover('destroy');
  }
});

// change mouse cursor when over marker
map.on('pointermove', function(e) {
  if (e.dragging) {
    $(element).popover('destroy');
    return;
  }
  var pixel = map.getEventPixel(e.originalEvent);
  var hit = map.hasFeatureAtPixel(pixel);
  map.getTarget().style.cursor = hit ? 'pointer' : '';
});