After the Black Omen appears, you can visit Ozzie's Fort in 600 A.D. and get some new equipment there.
Go to the Fort
Put Magus in your party and go to 600 A.D. Ozzie's Fort is east of Magus's Lair. Land and go inside.
Enter the Fort
Magus and Ozzie will talk. Go north to the next area.
Go north again, and there will be a battle. Use physical attacks to defeat Flea.
Go north to the next area. After the cutscene, keep going north and you will battle Slash. Use physical attacks to defeat Slash. Afterward, go north.
Don't step under the falling blade. Go northeast toward the exit, but before you leave, go south from the stairs. When hidden room appears, pick up the Magic Tab in the right side of the room, then open the three chests to get a DoomSickle, a Gloom Helm, and a Gloom Cape. Leave the hidden room and go toward the northeast exit again. After the cutscene, open the chest in the northwest to get a Full Ether. Don't exit the room yet.
The recommended party for the upcoming boss battle is Robo, Ayla, and Magus. Equip Ayla with the Charm Top, and equip everyone with armor that reduces or absorbs fire, such as the Ruby Armor from the trading hut in 65,000,000 B.C., and the Red Mail and Red Vest from the sealed chests in 600 A.D. and 1000 A.D. When you are ready for the battle, go through the northeast exit.
Great Ozzie, Super Slash, Flea Plus
Don't attack Ozzie while all three enemies are there, or they will hit you with a counterattack. Keep your HP above 250. Charm both Slash and Flea until you get Slasher 2 (a weapon for Crono) and Flea Vest (an Accessory that boosts Magic Defense). Keep hitting Slash with physical attacks until he is defeated, then defeat Flea (who has a chance of running away), and finally, defeat Ozzie. Ozzie will do nothing once his henchmen are gone. You can Charm him to get OzziePants, but it is a helm that causes Chaos and reduces your HP, so it is not particularly useful.
Open the chests to get a Dash Ring and a Sight Cap, then put Magus in your party if he isn't there already, and go north. Go north again, and during the battle, you can optionally Charm Ozzie again for another OzziePants. Otherwise, hit the switch behind Ozzie. Go north again and watch the cutscene.
If you go to Medina Village in 1000 A.D., everyone will be nice to you.