These sayings miss the whole point.
The idea is to buy some thing at some price and sell it soon enough --the same thing-- at a higher price soon enough. Knowing the time frame is as important as the % return because earning 19% in one month is the same as doubling your money in four months (re compounding). Knowing oil prices are down --by itself-- means little; we've got to also know how high the price is rising and how long it will take to get there. There are ways of finding this stuff out but it's a lot of work and I haven't seen anyone do it here yet.
If I could accurately predict the markets, I wouldn’t be posting from my home office now. I would be doing so from my 300’ yacht off the coast of France, or the Virgin Islands, or some such.
I doubt very many people predicted how fast and how much oil would fall these past weeks.