Dr accidentally re-used a thermometer without the plastic cover (noticed afterward), it this a risk for hiv transmission?? oral thermometer

No risk, no worry: HIV is not transmitted by oral contact. For example, kissing carries no risk, and neither does oral sex (or so little that there are no proved cases of HIV transmission mouth to penis). So even if a previous patient using the thermometer had HIV, you would not be at risk. Maybe very small risk for more common viruses, like colds and influenza. But not HIV.