DIMAP/NODE and ODE seem to treat this preposition differently; in ODE it is recorded simply as a variant of “on to” (treated at on) with an extensive usage note. Since there are FrameNet instances I broke onto down into the senses of on that it is identical with in ODE. I also defined an additional sense, in light of the FrameNet instances and some other corpus samples of onto usage that I looked at. ODE doesn’t really do onto justice as a preposition, since in Brit usage (for now, anyway, and according to their view), it is more often spelled as on to, but its usage is distinct from related senses of on alone. US dictionaries give a fuller treatment that more accurately reflects usage.
Notes from the spreadsheet:
Since DIMAP/NODE seem to have only one sense and this is not a sense that is defined in ODE, essentially all senses are new.
Sense 1(1): same SRType as on 5(1d), though usage varies slightly (see below under Other notes). This sense accounts for the overwhelming majority of FN instances.
Sense 2(2): same as on 16(7b)
Sense 3(3): SRType: ThingGrasped. Onto is the particle of choice for making several verbs of hold more compelling (hold, clutch, latch, grasp) when in many cases the verb will parse without its presence, or the verb would parse with to (bolt the pylons to/onto a concrete base). When onto is present the logical analysis is either as a phrasal verb with onto as the particle, or as verb + prep onto + complement: hold onto the post, latch onto his arm, etc. The essential idea here is attachment, and nearly all of the POAs for this sense are verbs of attachment and grasping.
Other instances of onto that may deserve idiomatic treatment
Cases where onto should genuinely be spelled as on to but is not are frequent and these are likely to be picked up as onto PPs: The police are onto him (should be on to), Let’s move onto the next point (should be on to). The one case in FN where this occurs is tagged as “on to” rather than with a sense number.
I have assigned onto the same SRType as corresponding senses of on because I think for our purposes they are equivalent, but it should be noted that there are differences in usage, and specifically in the case of sense 1(1), which is the main and most frequent sense, native speakers recognize a difference between on and onto. Consider these pairs of sentences:
Sentence A can mean the same as B, but most speakers would use B to clarify that he had been elsewhere but is now on the bed. Sentence A, on the other hand, can mean the he jumped repeatedly on the bed; sentence B cannot have this meaning.
Sentences C and D have the same meaning, but most speakers would regard D as the natural choice, and C as a slightly odd alterative. The same is true for sense 2(2) of onto, where there is equivalence with on, but where native speakers prefer onto when a verb of self motion is present:
We got on the bus.
We got onto the bus.
We stepped onto the bus.
*We stepped on the bus.*