First, in terms of word-searching, you can try and look up for 어미 '-오리다'.
-오리다² 【어미】(←종결어미, conclusive ending)
「참고 어휘」: -으오리다 (←이형태)
((받침 없는 용언의 어간, 'ㄹ' 받침인 용언의 어간 또는 어미 '-으시-' 뒤에 붙어))
「1」(예스러운 표현으로) 하십시오할 자리에 쓰여, 동작이나 상태에 대한 추측이나 견해를 나타내는 종결 어미
「2」(예스러운 표현으로) 하십시오할 자리에 쓰여, 자신이 할 동작의 의향이나 약속을 나타내는 종결 어미
- 제가 일을 맡아 하오리다.
- 오늘은 일찍 가오리다.
▲ allomorph: -으오리다
▲ antique style conclusive ending; attaches to a yongeon's stem that ends in a vowel (=no 받침) including '-으시-' or in a 'ㄹ'.
▲ sense 1: in an occasion to go hasipsio (=in hapsyo/hasipsio-che), it expresses a supposition or a view/opinion on an action or state
▲ sense 2: in an occation to speak in hasipsio, it expresses an intention or a promise about an action that one will take
And I presume you know that -으- is a 매개모음 (bridging vowel, epenthetic vowel), correct?
Yes, your guess is about right.
There are allomorphs -옵-, -으옵-, -오-, -으오-,
"(예스러운 표현으로) 서술이나 의문에 공손함을 더하여 주는 [선어말]어미"
TL: (as an antique style) an [pre-final] ending that adds politeness to speaker's statement or question
and as you probably already know, when you look up -리다, the definition is pretty much the same as the form with -(으)오-.
so you may say "지키오리니" is either
a)지키다 + -오리다 then conjugated with connective ending *-니- * or
b)지키다 + bunch of endings = 지키- + -오- + -리- + -니
also, out of three kinds of 경어 (존경-공손-겸양), the -옵- series expresses 겸양* (and indirectly 공손*)
*공손(법): "politeness". honorifics toward the hearer. This is aka 상대높임법 (addressee honorification) in school grammar. (this is where speech levels belong)
*겸양(법): "humblness"/"humility". AKA 객체높임법 (object honorification, in sch gmr) or 자기낮춤 (self-lowering, humbling). Indirect honorifics by humbling oneself or respecting a thing that's a part of or very closely related to the referent or an act/thing given toward them. (e.g. 드리다, 모시다, 말씀, 따님, 진지, 연세, 주무시다 etc)
this being said (tmi incoming), the series is eventually related to and cognate with -삽- series as well as -잡- series, and also -습- of 합쇼체 어미 -습니다.
(e.g. 성은이 망극하옵니다, 전하를 뵈옵고 (-옵-); 점포를 이전했사오니 (-사오-); 안녕하시와요 (-오-); 당신을 믿사옵고 따르옵니다 (-사옵-, -옵-); 말씀을 듣잡다/듣자오니; 밥을 먹삽고, 저희가 그 사람을 믿었삽더니)
so, as you see now, -오- in "지키오리니" and -옵- in "마옵소서" are the same thing: used for humble sense.
p.s. (another tmi)
btw, it's interesting that -리다 morphologically looks like 해라체 or 하라체 for it has zero honorific morpheme applied, but speech level-wise it belongs to 하오체, thus its politeness level is slightly addressee raising (예사 높임) …which is similar to the case with -ㅂ시다 ― morphologically appears to be 합쇼체 (아주 높임, greatly addressee-raising) but actually belongs to 하오체; although a clear misinterpretation since a) its -ㅂ- has nothing to do with -ㅂ- of 합쇼체 and b) its -시- has nothing to do with -시- of 존경/주체높임. Not of the same origin. Just a coincidence.