Should You Buy a Stock at RSI 30?

One of the first things most traders learn is when the 14-day RSI drops below 30, the asset is considered "oversold," and oversold means buy. The logic is mean reversion. Price has fallen too far, too fast, and is due to snap back.

We put it to the test properly. We didn't just check if it went up, because in a long bull market almost everything goes up, but "did buying at RSI 30 beat simply buying on a random day", across the S&P 500 and ten of the largest single stocks. The answer splits cleanly along one line: it works on the index, and it backfires on single stocks.

On the index, oversold is a real (if small) edge

	Buy any day	Buy at RSI 30	Edge
S&P 500	+1.4%, 66% up	+2.9%, 73% up	+1.4pp
Nasdaq 100	+1.9%, 66% up	+1.5%, 56% up	-0.4pp

On the S&P 500, buying the RSI-30 dip returned a median 2.9% over the next month and finished higher 73% of the time, a clear step up from the 1.4% you would have made buying on a random day. A diversified index cannot go to zero, so when the whole basket gets pushed to oversold it is usually broad fear overshooting, and it bounces. (The more concentrated, tech-heavy Nasdaq was already a wash, a hint of what is coming.)

On single stocks, it stops working, and on NVDA it backfired

Stock	Buy any day	Buy at RSI 30	Edge
TSLA	+2.2%	+4.5%	+2.3pp
GOOGL	+1.9%	+3.6%	+1.8pp
META	+2.5%	+4.3%	+1.7pp
MSFT	+1.7%	+2.0%	+0.3pp
AAPL	+2.6%	+0.9%	-1.7pp
NFLX	+2.7%	+1.3%	-1.4pp
AMZN	+2.6%	+0.3%	-2.3pp
NVDA	+3.1%	-6.2%	-9.2pp

It is a coin flip. Three names rewarded the dip-buyer (TSLA, GOOGL, META), four punished them (AAPL, NFLX, AMZN, and worst of all NVDA). Pool the eight together and buying at RSI 30 returned a median 1.5% over the next month, below the roughly 2.4% you would have made buying these same names on a random day. On a single stock, "oversold" is, on average, a slightly worse-than-nothing entry.

And then there is NVDA. The single best-performing large stock of the era, the one a buy-the-dipper would most want to own, is the one where buying at RSI 30 lost the most: a median of minus 6.2% over the next month, higher only 36% of the time. NVDA's oversold moments were not bargains. They were the middle of its most violent drawdowns, late 2018 and the whole of 2022, when the stock had already fallen hard and the knife kept falling. Buying it "oversold" meant catching that knife, over and over.

The trend filter does not save it

The usual fix is to buy oversold only when the stock is still in an uptrend, above its 200-day average, so you are buying a dip rather than a collapse. It helps for some names and hurts for others, and across the eight single stocks it simply washes out: oversold-in-an-uptrend returned a median 1.4% over the next month, oversold-in-a-downtrend 1.5%, all but identical.

Why the same rule means opposite things

RSI does not know what it is pointed at. On the S&P 500, an oversold reading aggregates 500 companies, and it takes broad, indiscriminate selling to drag the whole basket down there, the kind of fear that overshoots and reverts. On a single stock, an oversold reading usually means that one company has a specific problem, an earnings miss, a guidance cut, a broken thesis, and the market is busy repricing it. That is precisely the situation where the decline is justified and tends to continue. Diversification is the whole difference. It is what turns "oversold" from a reason to worry into a reason to buy.