陸 should be read りく not おか。
But in some cases like what you see, they alter the reading intentionally so the reader feel somewhat odd.
女 is read ひと instead of おんな for example.
There are that sort so many and I am not very proficient on those.
We've learned the standard reading at school. We've learned that "陸” is read as "riku" at elementary school.
So I think "riku" is the basic reading, as Wathavy said.
But in our daily life, they're interchangeable, depend upon each individual's preference, because the meaning is exactly the same.
In some idiomatic phrases, it would be read as "oka".
In other expression, it is interchangeable and depends on preference.
It is said that 2 different entry methods were used when we started to use kanji letters in Japanese society.
1. To use kanji as a similar pronounciation symbol. for example '左官- さかん：a person who make walls from mud and wood'. There are no meaning correlation.
2. To use kanji as a similar meaning symbol. for example '経済- economics'. There are no correlation of pronounciation.
陸 is a counterpart of 海 In formal writing, but it feels a little bit unrelaxed in usual conversation, and I use 'oka' in that relaxed situation as a conterpart of 海. This 'oka' is not a original meaning of '岡(small mountain)', but a meaning of '陸'. So, I feel '陸(oka)' kanji word is better fit in that situation as a counter part word of '海.
This is a No2 entry case of kanji. I don't feel much discontent in this using.
True, a well-known dictionary deals with the word oka in two entries (「大辞泉」 s.v. おか【丘／岡】 and おか【陸】), but the listed senses are clearly manifestations of a core meaning in different contexts. I think oka, no matter where it is used and how it is represented in the Japanese scripts, means a huge landmass. When used in a context that puts it contrary to the ocean, it shares the semantic field with the Sino-Japanese word/morpheme riku and written out as 陸. In reference to a geographic elevation on top of a flat land, oka is close to 丘 and 岡 and represented by them in written forms.
Now, Sino-Japanese words/morphemes are very active part of the Modern Japanese vocabulary in that they form numerous words loaned from European languages. In case of riku (陸), we have:
大陸 (tairiku [great land: continent])
陸軍 (rikugun [land corps: army])
陸路 (rikuro [land route])
着陸 (chakuriku [arriving on land: landing])
水陸両用車 (suirikuryōyōsha [water-land both-used wheels: amphibian (vehicle)])
It is a morpheme for technical terms and oka is a word for everyday life. No wonder old sayings such as the one Wishfull quoted above use oka instead of riku. Nowadays the general term for "land as opposed to sea" is riku, due largely to many technical terms, but oka is frequently used in this sense by those whose daily life has a lot to with sea.
furrykef, if the author of your text prefers oka to riku, maybe the speaker of the sentence is someone who knows ocean very well, such as a fisherman or a sailor.
I seem to understand you are saying that the script representation 陸 is misleading because the saying uses oka. The same force of tradition that has passed it on to our time, however, allows oka to be written as 陸.