He and Paisley were one stroke in front of Spain's Rafael Cabrera Bello and Thailand's Kiradech Aphibarnrat, both in the clubhouse on five-under 66.
Paisley birdied his opening hole at the par-71 layout, which is more than 7,500 feet above sea level at its highest point, before draining a 30-footer at the second and posting back-to-back gains from the fifth.
Johnson, for example, had two bogeys and a double-bogey and played the par 5s, which he feasted on previous year on his way to victory, in 1 under par. "I've been swinging it pretty good the last three, four weeks, felt I played really well last week. The way I hit it, I probably should not have shot 2 under".
Paisley's himself praised this new rising star Cameron Paisley.
This is only Oosthuizen's second PGA event of the season.
"It's the biggest tournament I've played in by a long stretch", the Briton said.
"I gave myself a lot of chances but they're tough greens to putt on and to hole as many putts as I did, I'm delighted".More news: Smith & Wesson Parent Cuts Forecast, Sees Weak Market Ahead
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Last week's Honda Classic victor, Justin Thomas, never found his rhythm in the opening round and signed for a 1-over 72to be T39.
Paisley held the outright lead after following a front nine of 31 with birdies on the 10th, 14th and 15th, but bogeyed the 16th shortly before Oosthuizen added an eagle on the 15th to five earlier birdies to vault to the top of the leaderboard.
Paisley won the South African Open in January - his first European Tour victory - and followed up with consecutive top-five finishes against strong global fields in the Middle East. "I really kind of started on Tuesday, wasn't swinging it very well, and then yesterday I just was hitting it bad".
Victor of the 2010 Open Championship, South African Oosthuizen opened with a seven-under-par 64 in Mexico on Thursday.
World number two Jon Rahm is three off the pace after a 67, with fellow Spaniard and Masters champion Sergio Garcia a shot further back.
The billboards around Chapultepec understandably include the likes of Johnson, Spieth, Fowler and Thomas.
But then, while it might be billboards that sell tickets and generate interest, it's the names on the leaderboard that count once the competition has started, whether that lineup includes a superstar cast or not.