It's essential not to rely solely on the RSI to determine where to buy a stock. It should be used with other indicators to form a solid buy or sell thesis. The good RSI buy signals form when the RSI oscillator rises back up through the 30-band. However, it is crucial to be aware of the trend of the stock as well as the context for the sell-off. The selling is often not related to the individual stock but because the whole market itself is selling off.
For example, the S&P 500 index selling off on a high consumer price index (CPI) number will likely impact the RSI on your stocks to fall as well. Be sure to check the benchmark indexes just to be sure. Be aware that buying a stock with an RSI of 70 or higher has the risk of chasing a price that may exhaust and reverse as soon as you get in.