You can book codeshares on OneWorld Explorer fares, provided both the flight number and operating airline are OneWorld alliance members.
However, there are often restrictions around the sale of codeshares. It may be restricted to a particular market geography (ie only sold in North America and not in Australia), or may only be sold in conjunction with a specific airline's connecting flights (ie you may have to arrive in an AA flight to book the connecting AA codeshare). These are not OneWorld Explorer rules, but specific rules for the particular codeshare flights.