Unfortunately, mushrooms don’t fruit well off of just grain. Though some mushrooms may eventually grow off grain spawn, the yields will be minuscule. It's much better to use it to inoculate other substrates.
Most mushrooms prefer to fruit off of grain mixed with substrates like manure, hardwood, sawdust, compost, or straw. (Fun fact, oyster mushrooms will grow on all of these! )
If you are looking for an easy way to grow mushrooms indoors, try starting with a Spray & Grow Kit or Fruiting Block Kit. They are fully colonized on amended sawdust blocks and ready to grow.