Girona's 1-0 win at Cadiz temporarily gave the Catalan minnows a one-point lead but Madrid cruised to victory at the Santiago Bernabeu to reclaim first place, with Vinicius Junior and Joselu also scoring in the rout.
England international Bellingham, the division's top scorer with eight goals, was at his unstoppable best once more in the final third.
Real Madrid coach Carlo Ancelotti has shifted the former Borussia Dortmund player into a more attacking role behind two central strikers and it has paid dividends.
After scoring a brilliant solo goal against Napoli in midweek in the Champions League and adding two more against Osasuna, the midfielder now has 10 across both competitions in 10 appearances.
"He's in top form, he's showing the player he is, I hope he keeps on like this," Madrid defender Dani Carvajal told Real Madrid TV.
"I hope he keeps scoring goals and keeps helping us, because he can win us titles."
Ancelotti deployed defensive midfielder Aurelien Tchouameni alongside Antonio Rudiger at the heart of defence, with David Alaba and Eder Militao injured, and Nacho suspended after his rash tackle last weekend against Girona.
However Tchouameni enjoyed a quiet afternoon at the back as Madrid dominated to ensure they will stay perched at the top over the international break.
Madrid broke the deadlock after nine minutes when Luka Modric, afforded a rare start, fed Carvajal who laid the ball off neatly to Bellingham.
The midfielder, who had surged into the box, opened up some space and then blasted home, leaving goalkeeper Sergio Herrera with little chance.
Vinicius fired narrowly over when he might have passed to Bellingham, with Madrid goalkeeper Kepa Arrizabalaga reduced to a mere spectator at the other end.
Osasuna did carve out one good chance, shortly before half-time, with Ante Budimir slashing over the bar after being set up nicely by Ruben Pena.
Bellingham brace
Bellingham started and finished Madrid's second goal in the 55th minute, darting forward and playing a one-two with Fede Valverde, before sweeping home, showing the combination of grace and dynamism he is known for.
Vinicius extended Madrid's lead when Valverde sent him running through on goal, with the Brazilian expertly rounding goalkeeper Herrera and tapping home.
The winger turned provider for the fourth goal, setting up on-loan striker Joselu, who gleefully finished for his fifth league goal in his last six games.
"It's a tough place to go, against one of the best teams in the world -- they were far superior," Osasuna wide man Pena told Movistar.
Joselu's sixth should have followed when David Garcia conceded a penalty for handball, but Herrera saved the forward's effort, fired down the middle.
Osasuna's goalkeeper has a superb penalty-saving record, including stopping two efforts from former Madrid forward Karim Benzema in one match.
Joselu's miss did not take the shine off Madrid's afternoon, as they celebrated their most emphatic win of the season to date.
"The team came out for the three points from the start, we played a complete game in all areas, and we gave up few chances," added Carvajal.
"We achieved our objective of reaching the international break as leaders."
