Your MapProxy configuration

caches:
  osm_cache:
    grids: [GLOBAL_WEBMERCATOR]
    sources: [osm_wms]
grids: {}
layers:
- name: osm
  sources: [osm_cache]
  title: Omniscale OSM WMS - osm.omniscale.net
services:
  demo: {}
  wms:
    md:
      title: Default WMS
    srs: ['EPSG:4326', 'EPSG:3857', 'EPSG:900913']
sources:
  osm_wms:
    req:
      layers: osm
      url: http://osm.omniscale.net/proxy/service?
    supported_srs: ['EPSG:4326', 'EPSG:3857', 'EPSG:900913']
    type: wms