As a side effect of the way damage spells work for Onn (namely uncapped for scaling damage magic), at some point a character who is only gaining a flat +1 or +2 per level is going to get wiped out by something like a high level caster's fireball, even if the saving throw is made.
Going back to OD&D, the classes did continue to gain HD after they reached Name Level (8th for Clerics, 9th for Fighting-man and 11th for Magic-users). Its something that the more modern (Supplement I Greyhawk on up) versions of D&D have eschewed in favor of caps on spells and lower HP progressions until the sunset of 3rd Edition/d20 RPGs. They went super powered though, with big dice and every level adds.
I went back to OD&D and looked at the patterns and Onn and looked at its patterns as well and came up with the following general changes to the HD progressions:
|Character Hit Dice|
Now at the levels the character does not gain a HD, the character gains +1 hp with Constitution adjustments (minimum of 1 hp). Onn uses HD to determine immunity to spells and spell effects (like Holy Word or Death Spell) so using a scaling HD also makes for good sense for characters as well as monsters.