Learn English – What’s the sentence pattern of passive voice

The two sentences under question are

• We saw him enter the room.
• He was seen to enter the room.

The second is clearly the passive transform of the first. The problem is what to do with the infinitive complement, complicated by where the to came from (or went to, if one uses a different model). As usual, a great deal has been deleted and modified already in that first sentence. Where did it come from?

As I mentioned in the comment, that first sentence is the result of B-Raising, followed by Passive.

If you don't understand right away what I mean by Raising (never mind A- or B-), you can take a look at the link. It's some explanation, with examples, and a couple of solved problems. This is real English syntax, and it's likely to be strange; sorry about that. Strange people like me and my students think it's interesting.
Or you can try to make sense out of what follows. Or, preferably, both.

The sense verb see governs B-Raising, which requires an infinitive object complement clause for see, roughly -- pre-deletions -

• We saw [[for him] [to enter [the room]]]

The infinitive object clause [S] has inner bracketings for the subject [NP] of the infinitive clause and the infinitive verb phrase [VP] of the clause -- which contains a direct object NP. Both the subject NP and the VP are marked by complementizers (respectively, for NP and to VP). I.e,

• *We saw for him to enter the room.

  Of course that's not grammatical. Yet. Because all the required rules haven't applied. Yet. Intermediate structures with complex rule interactions are often ungrammatical. But like imaginary numbers, they're useful and they never really occur, so it's OK.

See is a sense verb and therefore occurs in a lot of idioms and metaphors. Mostly they involve making reference to metaphors, like I see what you mean. And most of them involve chopping out chunks of clauses ("clause reduction" as it's called in the trade; see Jim McCawley's famous paper "On Examining the Remains of Deceased Clauses").

It is normal for the subject complementizer for to be deleted, unless the infinitive is at the beginning of a sentence.

• I want him to win the trophy.

• %I want for him to win the trophy.

• For him to speculate would be premature.

• *Him to speculate would be premature.

  This is especially true when the subject gets deleted with Equi. So the for in [for him] is normally lost -- just remember that complementizers can surface again, in different constructions, and different dialects; this is where for to see my Pollyanna comes from.

Another of the peculiarities of sense verbs is that they don't allow to with an infinitive complement.

• *We saw him to enter the room.
• We saw him enter the room.

So both of the complementizers are deleted, leaving only the bracketing

• We saw [[him] [enter the room]]

That's still a direct object clause. What Raising does, when it's governed by a Raising verb like see, is to change the bracketings, freeing the [him] to be the new Object of see, with the infinitive clause, now demoted to a phrase (en chômage, in Relational Grammar terminology), left right where it is at the end.

• We saw [him] [enter the room].

Now Passive applies to this Raised object him, producing

• [He] was seen [to enter the room].

Except that the passive of see doesn't have quite the same affordances as the active -- as I said, sense verbs are complicated -- and so the passive seen still requires the ordinary to complementizer. Which is automatically provided, free of charge, like the do of Do-support, from That Big Bag Of Auxiliaries In The Sky.

P.S. Linear structure mashups like “S+V+O+OC” are very idiosyncratic, are not good representations of what's really going on, don't contribute much information, and therefore probably are not the best way to search for information. Syntax is constructions and rules.